They know their playerbase. If they do that, everybody will only want to use stacks into ED above anything else, and not long after you'll see people coming on the forums and complain that it sucks that their healing conflicts with an attack "used for 2 minute bursts" and that it makes them lose damage if they use them for other things.

With which other role I have heard this hmm... oh yes, the blue one on gap closers.