The nerf won't be utterly useless in PvP. Even with high crit resistance, critical hits still do more damage than non-critical hits, especially since some gankers use the Shadow Stone and CPs into crit damage to counteract Impenetrable.

With that being said, however, I have a pretty strong feeling that this nerf is directed more at PvE than PvP. It's clear that with this update, ZoS wants to help bridge the gap between crit builds and non-crit builds. Proc sets, in the current update, use only crit and none of our other stats, which means that if you're running Grothdarr or Ilambris or what have you, then crit is absolutely the best way to go, since you can't build Magicka or Spell Power and simultaneously boost your proc set. So even if this change isn't necessary for balance, it is necessary for consistency and build diversity.

As far as balance goes... Well, Grothdarr and Ilambris are still most likely going to remain best in slot for Magicka DPS builds. This isn't going to straight-up ruin PvE, or even most endgame builds. It's barely even going to shake up build diversity. And even if it would have earth-shattering ramifications for PvE, I'd say that completely justifies the change, since that means the entire weight of PvE rests on armor sets that are almost completely independent of player interaction.