Originally Posted by Talaii

Warlocks damage (unless destruction) is based around dots. One of these, a fairly major one, has a 30-second duration. If fear broke on any damage, a warlock would have to decide a good 30 seconds in advance (at least 18, since they'll be using corurption and/or UA) who they were going to fear, and even then, one miscast onthe wrong target will render that person almost immune to fear for a good 30 seconds.

If you're going to make fear break on any damage, you at least need to give an affliction warlock some method of removing dots - the cost of leaving someone open to fear would simply never be worth it, at least as a defensive option. Most other classes have defensive options that aren't completely negated for long periods of time by you casting a single spell.