Why? A soft enrage that changes the mechanics of the room can be just as 'impossible" if they really need to impose a hard limit on fight duration. Let's say Guldan enraged at X minutes, a hard enrage where the room just blew up and one-shot everyone. What's the difference between that, his next outward-growing AOE attack (I can't remember the name) covering the entire room instead of just half, or having 75 souls spawn in the middle? All three situations are pretty much a "you're screwed" to the raid but at least they're interesting and maybe, just maybe, could be survived for a few extra crucial seconds.
Does it really make a difference if a raid-wipe mechanic looks one way or another?
^nope so hard or soft, it really doesn't matter imo.
our monk tanked spellblade's enrage for about 30s the other week on mythic (because brewmaster ho ho ho)