That number probably doesn't seem like a lot to people who don't set foot into the arena at all, but for people who arena seriously and competitively, it will be a game changer. The developers' stated goal is to slow down the pace of the arena, and this change will do exactly that. An extra 10% buffer on survivability has the potential to absolutely be the difference between a one minute game and a five minute game. It has the potential to be the difference between a gib and a last second defensive save. As has been stated, it's possible to die in the span of one GCD, and that's not fun. If this change can extend one GCD to two or three GCDs to allow for reaction time, the arena just might end up more balanced than it has ever previously been, unless you pretend Mace Stun didn't exist in the middle seasons of The Burning Crusade.
You really shouldn't take my word for it, though. I PvP, but I'd never claim to be a highly rated anything. There's a reason I was waiting for Ming's take on it. Go read what he has to say, and be bolstered by it. The arena might actually be truly fun again.