Its impossible to find places to put the unique demon abilities when you're action bar is already full +needing spaces for all those other new skills and talents. And since you usually can't even use them they are just wasting space when you aren't in Demon form.
Also - most of the abilities aren't much good. Charge is good... immolation seems alright. But Shadow Cleave does like 200 damage and aoe taunt is very situational - probably emergency in a 5 man instance when the tank dies or something.