Incubi are a bit of an odd option in Legion, I find. It's a bit hard to stomach 5 points that basically don't appear on the board until things start dying. However, I've really been thinking about them, and the more I consider them, the more things I've realized that make them sound more and more respectable for their cost.
- They aren't really 'not there' at the start of the game. Your opponent knows they're in your list, and knows that they'll pop up from a unit, this means he has to consider Incubi could pop up anywhere a legion warrior model dies, so they're causing a psychological effect right from the get-go.
- They're the perfect second wave models. Many factions have second-wave models. Slower, harder-hitting, often not too durable. They're meant to advance behind a screen, and counter-charge when that screen gets engaged. Incubi do exactly this, except they're invincible until they want to pop up, they're always going to be in counter-charge range, and they're a swarm of solos instead of a coherent unit (has its ups and downs). Best of all; you don't have to worry about protecting, buffing or moving them until it's time for them to get into the thick of things.
- They cut through enemy lines like it's no one's business. Being placed within (not completely within) 2" of the point of death is a fairly awesome deal. That's plenty of room to spawn behind units (behind heavies, even) then proceed to charge the soft underbelly of an army, or be placed facing the backs of whatever they spawned behind, then charge them in the rear. Backstrikes making them an effective MAT 9 is just awesome.
- They can cross the board. This can be surprisingly difficult for most second-wave units, but the Incubi are right there, literally ontop of whatever forward screen you have, and aside from just being able to counter-charge, they can just take off on a full run afterwards to go wherever you need a model to be.
- They're actually pretty versatile. Need accuracy? spawn for a backstrike. Need damage? Combo strike. Need volumes of attacks? Make both attacks. Their MAT 7 PS 11 is a great starting point for their variety of tools.
- They're very likely to work in your favor. There's the notion that they're risky because your opponent just might not kill things, but really, though, look at some of the units that are ideal front-line troops; Hex Hunters, Legionnaires, Swordsmen. These are all solid units. If your opponent doesn't kill half the squad in one fell swoop, then any of those units are going to massacre things in retaliation for getting close enough to try. If they do kill half the unit, then boom, there's all 6 Incubi, like a second wave just spawned in the middle of the fight wherever they're needed most. Arguably, a single Incubi dropping into the middle of the fight with the potential for charges and backstrikes is potentially more deadly then most other single grunts would be. So if anything, the loss of 6 x-troopers to allow 6 Incubi to charge in is usually going to be awesome. Even if they started the game in play, they wouldn't get into combat until a bunch of things had died anyways.
So those are my thoughts on Incubi, which have currently lead me to put them at the top of my next-to-purchase list, and considering them in any list where I have a first-wave unit.