Last night in my 13th Age session I made extensive use of Spoiler type monsters for the first time. Specifically, an annoying flock of Harpies. When I first read the Harpy stat block I thought "oh cool, Fiendish Song has an escalating series of effects for higher attack rolls!" Then I realized that the chances of a single Harpy rolling a natural 16+ weren't necessarily very high if the thing only lasted 4 or 5 rounds. This comes from a habit in other systems of using "debuffer" or "controller" type monsters as a single creature allied with (usually) some frontliners or skirmisher types. But they usually got attacks that generated a given effect if they hit. These Harpies usually just deal piddly damage until they get lucky enough to start messing with you.
I almost immediately decided to throw 4 Harpies into the encounter. If Fiendish Song is getting rolled 4 times per round (often against multiple targets) then those extra effects for high rolls are bound to come up! Ultimately I like to showcase the abilities that make a given monster unique, and in this case it's the ever-increasing nastiness of the song. If those effects never go off it doesn't evoke the feeling of fighting Harpies nearly as strongly.
So use groups of monsters when they have difficult-to-trigger abilities. This doesn't only go for Spoilers or Casters either. The first time I used Lizardfolk (in my other group) I didn't use enough of them, and I rolled low. The party had no idea how nasty it could be when a Lizardfolk lands a good enough blow to open its opponent up for a Bite attack, nor the carnage that ensues the round after the thing is able to clamp its jaws down onto you. That's an important part of the Lizardfolk's fighting style in my mind, and evokes strong imagery. Yeah they use weapons (albeit primitive ones), but those weapons are mainly for punching through their opponent's guard and getting into extreme close range so that it can use its natural weapons; a nasty vice-grip bite and a barrage of scratches once the foe is held down by the bite.
Despite that being the Lizardfolk's schtick, I like that the combo is designed to come up infrequently in combat. This allows the effects to be stronger than they otherwise would be, and it generates more drama. Normally if the natural attack roll is high but not a crit it doesn't really matter; as long as you beat AC it's all the same. This way it doesn't become monotonous or annoying. On the contrary, it's memorable because it really challenges the PCs in an evocative way, and each subsequent die roll carries an added tension ("oh crap, I really hope he can't snap in close enough to bite me again!").
Grouping such enemies not only makes their abilities more likely to be triggered during a given fight, but it also creates more tactical opportunities. Because Harpy talons are nastier when the target is affected by the song there's a synergy that's created and the other Harpies have a legitimate reason to focus-fire. But that also creates an opportunity for the PCs because they need to swoop down to ground level to do so, making themselves vulnerable to melee attacks. The tactical landscape changes for both sides.
Even the Lizardfolk present similar effects. Focus-firing is always going to be a good idea if someone on your team has just landed a really big hit. The Lizardfolk can all start mobbing the guy that's just been severely injured. Because its random who becomes the target, players will also be less likely to feel "picked on." Creating an incentive for the NPCs to move also increases the likelihood of the PCs getting opportunities to intercept or to bring specific talents into the spotlight (the Barbarian's Slayer, the Fighter's Threatening, and Paladin's Challenge all spring to mind).
It can really be boiled down to the following: tough triggers allow for more powerful effects, increasing numbers boosts the likelihood of those effects being triggered, and powerful effects tend to have a big impact on the tactical landscape (making fights more exciting and dynamic). Fighting four Harpies and having players get Weakened a handful of times and Confused once during the battle is cooler than if the Harpies were able to just Hamper or Daze enemies more reliably (say, on a natural even hit).