Does this mean that PvP gear should be unusable in PvE instances?
Sounds unfair otherwise if high end effort in PvE doesn't have any effect in PvP, but PvP gear that naturally has an ilvl comparable to PvE gear is useful outside of PvP. High-end raiders have their trinkets and tier bonuses, and pvpers has pvp trinkets/pvp set bonuses.
See, you can't just exclude one side of the issue and pretend it's fair. Imposing unnecessarily strict rules of where gear is "useful" and how it compares to gear of that "subset of activities" doesn't bode well for gameplay for the vast majority of players... I don't see a high end mythic raider occasionally jumping into a rated BG to wreck face, his PvP "gear disadvantage" offset by tier bonuses and trinkets, is much different than a top rated gladiator jumping into heroic or even mythic raid because his ilvl and survivability offsets the lack of PvE-oriented bonuses