I am 100% using PvP to level other jobs right now. But I have an affinity for PvP and I contribute well to the team. If PvP didn't give experience then I wouldn't have the desire to try out other jobs like MCH, BLM, AST, or WHM. I power-leveled WHM and BLM to 30 for this purpose where I otherwise would not have.

Adding experience to PvP gives me options and a reason to PvP on other jobs. Now I can swap to healer if we need one, or ranged dps, etc.

I think everyone benefits greatly from me being able to drop bombs on BLM or swap to WHM when needed, instead of piddling around and sniping killing blows on my Dragoon. You can thank experience in PvP for that.