|Straight up murderous instincts right here|
22 points a piece introduces you to the idea of having a save that isn't ignored by a stiff breeze. The only unit outside of a monstrous creature to feature the much feared and coveted 3+ armour save, Incubi can roam the battlefield without much fear for their saves being ignored - but the enemy will make them a high priority target. Every game I've used them, they always get shot down and killed, but it just lets my Wyches or other units go unmolested and do their dirty work. If you can drag a Homonculous into the mix, you can have a very resilient unit with 3+ and FnP.
These guys do have upgrades! A character with an extra attack, a sword that can either grant +2 attacks, or +2 strength, your choice. A S3 AP3 template, and a few powers. One that lets you re-roll to hit against Independant Characters, and one that allows you to roll an additional attack for each 6 you roll to wound with an Incubi. Can't get infinite attacks with them though, which is a real shame. How come that Blood Angels dreadnought can do it, but they can't? Weeeeak!
I think for their cost, they're one of the most cost-effective units in the codex - paying for Klaivex is mostly just for show. A fully tooled out Klaivex costs 97 points. You can get an Archon with a Husk Blade for his cost! Which, you know, might be good if you already have two Archons or some silly shit like that, but rarely will it do you any good to have so many points invested in a one wound model, unless you're playing Grey Knights.