I've done moderation myself for forums, and I reckon that having a PvP-based mod (if it was the right person) would definitely help. Online communities flourish so much more when the posters get along with and respect the mods. Every mod should be impartial but the reality is that they are human just like everyone else and that doesn't always happen. If all the mods are PvE, you are going to have some bias towards PvE, even with the best of intentions. Furthermore, that obviously doesn't help foster the spirit of the PvP community as they don't feel represented.
If you have a mixture of PvP and PvE mods, all trying to be impartial, then any little biases are more likely to be balanced out, the mods themselves can challenge each other if needs be (this does happen) and most importantly, PvP players are going to feel better represented and respected and more likely to post and get involved.
Look at it this way, if a PvP player currently posts and gets moderated, it's all too easy for them to cry "bias" and no one can definitively say they're not wrong. When you have both PvE and PvP mods on board and someone cries bias, then you have a valid counter-argument in that they're now represented by both sides.
Just my 2 cents as a newbie
