I like Incubi as well. They are a little bit harder to use than Grots, but they usually get the job done, as long as you pick the right job for them. If you are lucky and play against someone with several small backfield units you can use incubi to beat them up and get some good mileage out of them.
Or as counter assault as mentioned.
In my last game I used them as counter assault. They pounced on some hammernators that tried to teleport in and ruin my ravager party and chopped them to bits without breaking a sweat. The glory was only slightly overshadowed by a unit of shining spears that wiped an identical squad of terminators the same turn and in the next turns went off to wipe a squad of tacticals, and then punched some holes in a sicarian tank. All this while my incubi scrambled back in the transport and then milled about looking for targets. My bad for bringing craftworld allies I guess...