I'm sure English ones would be just as good but by now we're all used to and even cherish the Russian voiceovers. Don't forget also each language has its unique, untranslatable "atmosphere" or "mood" - however nice English ones could turn out to be they would definitely take something away from the intangible Russian color to this game.
(I would like however personal names from most generic NPCs localized - obvious exceptions would be "translingual" names like Aidenus, Yasker, etc. and all the race-specific names except for the human Kanian and Xad ones that use vernacular naming.)