PvP and PvE should be separate gear, and itemized accordingly. This prevents the hardcore raiders from being able to completely dominate in PvP, because they have the best PvE raid gear. If you want to raid, go ahead. The gear rewards from doing so, should enable you and your raid to progress further into PvE content at a higher difficulty. PvP should be a level playing field. Eventually everyone will be 50, and those who PvP will all have their expertise gear.
Also expertise as a PvP stat makes it so that PvP is actually based on skill, rather than 3-shotting someone. You will have to isolate DPS from healers, use CC properly, and time out your kill shots on enemy opponents. This makes for better PvP battles, than say, stun-locking someone in CC and 3-shotting them without them having a chance to defend themselves.