If I see someone in Myth gear, I can safely assume that the person wearing it has probably run primals and dungeons a significant number of times to at least grasp some common battle mechanics in the PvE content to have acquired his gear. If PvP gave out Myth tomes, however, it would be more difficult to tell since now there's a chance that this same player may have put all his time into PvP and obtained his Myth gear that way, thus may be completely inexperienced in PvE party mechanics.

This is the same reason that I believe it's a good thing high level PvP gear is obtained from PvP only. Just imagine if Coil or Crystal Tower dropped PvP gear; there would be a chance for you to be matched up with someone decked in full Direwolf gear that just gets demolished by the opposing team out the gate because he hasn't gained the same level of PvP experience.