Jan 20th 2009 8:03AM Although as a high-rated, arena-experienced mage this is a nerf to me, I can see how blizzard came to this decision and I peronally agree with it. This change is purely for pvp, as people have mentioned previously, so anything about dungeons is kind of off-topic.
The reason why i can accept this change is that i'm a proponent of certainty and reliability competitive situations. The bigger the role RNG (thats the % based chance of things happening) has in PvP, the less purely skill-based pvp becomes due to many aspects of it relying on luck. Blizzard realized this by changing many talents and racials such as orc hardiness, which used to be 15% chance to resist stun completely, to reduce stun duration by 15%. The same goes for meta gems, and most significantly, cast times (talents reliably always reducing a certain amount of cast pushback instead of giving chance to resist all pushback completely)
With this design concept in mind, losing a game simply because a healer had bad luck dispelling due to that 30% chance to resist dispelling polymorph for example is a loss based on luck and not skill. Granted, blizzard fails to take RNG out of pvp completely, since crits and hamstring procs etc still rely on luck, but at least this is a step towards that direction and its effects will be noticed immediately.