I'll be frank: I firmly believe that Vancian casting (at least as it's existed in D&D up to and including 3.5E) is a sacred cow that needs to be put out to pasture for good. I could use a cheeseburger.
But let's back up a second. What exactly is Vancian magic? As defined by TV Tropes, it's basically a "fire and forget" spell-casting system where the caster has a finite number of uses of distinct spells. In the first 3.5 editions of D&D (as well as Pathfinder) this took the form of long spell lists divided into levels (that didn't correspond directly with character level), and spells had to be prepared ("memorized") at the beginning of each day by choosing which "slot" would be filled with which spell. If you have multiple slots of a given level available, you can prepare the same spell multiple times.
Technically 4E didn't get rid of Vancian magic, but made some major modifications from previous editions: 1) powers have different "recharge" times (some powers are refreshed after each encounter instead of each day), 2) normally you cannot "prepare" the same power twice, i.e. a specific power is married to a specific "slot," and 3) the modified Vancian "magic" structure was applied to all classes (even non-casters), which is the key to 4E's exceptional balance. Because this isn't traditional Vancian casting it's often not even recognized as Vancian, which is fine by me. The traditional method is what I have a problem with, and so needless to say I was very displeased to hear that it will return in 5E.
Even if non-Vancian modules exist to cater to alternative playstyles, Vancian magic is so inherently imbalanced that its very presence will likely ruin the game (even if just 1 player opts to use it).
The Fallacy of "Temporal Balance"
An argument that is sometimes put forth is that Vancian casters are "balanced" because they're weak at low levels and exceptionally powerful at high levels, thus everyone gets to have their time in the spotlight (albeit not at the same time). There are some pretty obvious problems with this argument. A campaign starting at level 1 and going up to the highest levels takes a VERY long time, sometimes multiple years. It's not fun for someone to play a weak (sometimes completely obsolete) character for weeks, months, or years "until they get better" or "knowing that they used to be better." The assumption of balance breaks down entirely when a group either starts at high levels (circumventing the Vancian caster's "weakness") or quits before reaching high levels (not letting the Vancian caster ever come into his own). I'd wager that this is the case for the vast majority of D&D players, and that only rarely is a campaign played from level 1 to maximum (20 or 30).
From a game design standpoint, "temporal balance" is not good balance; good balance is making sure that everyone is capable of making useful contributions at any given level, because it's the only way of ensuring that balance is relevant in play. In other words, if you never see certain levels then the fact that those levels provide a type of balance is irrelevant. Such balance might as well not exist (and in fact, at any given point in time except the narrow "sweet spot," balance doesn't exist.
The Five Minute Workday
Vancian spellcasting is supposed to facilitate a playstyle based on resource management, but in practice this often won't happen. The casters use all of their big-guns in the first fight of the day, and then force the party to rest so that they can re-gain all of their spells (either by retreating, or using Rope Trick, Mordenkainen's Mansion, etc). This makes them even more powerful than the designers assume (based on a multiple-encounter work day), allowing them to trivialize encounters and make the classes that favor consistent performance obsolete. It also makes the DM's job of designing appropriate challenges much more difficult.
Yes, the DM can set narrative limits such as a "race against the clock," but overusing such tools eventually strains suspension of disbelief, and besides the fact it's not the DM's job to work around imbalances, it's the job of the designers to ensure that they don't exist in the first place.
Fragile Balance, Easily Broken
Even if the design team, against all odds, manages to balance Vancian casting with other available options at launch, such balance won't last. Power creep is inevitable in any system as more supplements are released (after all, why would anyone buy them if they weren't worth using?), and even without intentional increases in power it inevitably becomes more difficult to maintain balance as more options come out. More options means more possible synergies and combinations, and it's intrinsically more difficult to balance a larger list of variables. But shouldn't this affect non-Vancian characters just as much as Vancian casters? No, because Vancian casters improve disproportionately with new and improved options.
Just look back to the 5 minute workday; the better spells a caster has the more he'll trivialize that first encounter, and the greater the incentives to actually pursue a 5 minute workday. Outside of that, consider a 4E Wizard vs a 3.5 Wizard. If a new overpowered Daily power is published in 4E, the Wizard can be that much more awesome once per day. If a new equally overpowered spell is published in 3.5, a Wizard can use all of his available spell slots to prepare it to be that much more awesome multiple times a day. A Vancian caster is also able (encouraged, even) to have a huge variety of spells that they can potentially prepare, putting them at an advantage when it comes to having a situational spell in their back pocket or utilizing synergies (even those with lower level spells that the 4E Wizard would have long since retrained). Assuming Polymorph/Wild Shape rules consistent with D&D 3.5E, even power creep among the monsters can be taken advantage of by Vancian casters.
As a final note, my confidence that the lead designers for 5E can achieve even an initial balance is minimal. Monte Cook was in charge of 3.0/3.5, arguably the most grossly imbalanced version of D&D, and even 4E's balance, with all of its safeguards, went downhill when Mike Mearls was put in charge. This isn't even the result of new sub-systems too different from the baseline class structure (indeed, classes like the Slayer, Scout, etc. are actually pretty well-balanced). No, I'm talking about the complete and utter failures like the Binder, Bladesinger, Vampire, and Cavalier. These largely follow the classic structure but have features (or a lack, thereof) that just blatantly reek of poor design.
Final Notes On Balance
I should clarify quite clearly here that I don't mean to advocate a naive notion of "perfect balance." It's impossible to achieve, but game designers should certainly try to achieve the best balance they can. Monte Cook, on the other hand, wrote an article (the discussion of which I won't even attempt to find on the WotC forums because it was a while ago, and the original article you had to pay for anyways) where one of his major points essentially came down to this: because perfect balance is impossible, it's pointless to even try to achieve balance, so you shouldn't make it a priority. What utter trash that reeks of someone trying to justify their own bad design.
I'm reminded by an episode of Malcolm in the Middle (at least I think so; I could be mixing up shows because it's been a while). Dewey was playing a game simply called "I Win." As I remember it, it was a card game. Regardless of what hand everyone got, Dewey would always win because that's the one rule of the game: "I win." Obviously this is at the extreme end of the balanced vs unbalanced spectrum, but I think it does a good job of illustrating the point that balance does matter. If a spellcaster can summon a creature that fights just as well in melee as a Fighter (or Polymorph or Wild Shape into one), and still retains the ability to cast spells, then playing a Fighter is an awful lot like playing "I Win" with Dewey. Sure, D&D is a cooperative game, but being completely unable to contribute is analogous to "losing." If your class is obsolete, what's the point of playing?
Such glaring imbalances actually existed in 3rd edition, and are seemingly defended by Monte Cook, the guy in charge of 5E. I don't have much hope that it will be a game that I'll enjoy playing, but we'll see. There will be a lot of really intelligent people playtesting this game, and hopefully Monte won't be too stubborn to reverse the direction of "his" game if there's demand for it.