as Anggul said
This makes no sense as all the other races have the same issues of casting psychic powers in battle as the warlock does but the warlock has the added disadvantage of being leadership 8 and only one wound. Also Warlocks are described as being battle psykers (at least in the last codex, given by they had no chance of failing and their powers were always active).
Pretty much Anggul nailed it on the head, just wanted to add my 2 cents