Once you offer rewards for doing it, then no matter what your original intentions where, it will be abused/exploited.
Allowing rewards for PvP (especially +4 tomes....REALLY??) would mean that every guild in the game would be running the PvP stuff to exploit it for the rewards, with very little risk. As I stated earlier, the mechanics for this kind of thing are ALREADY IN THE GAME, they just require YOU as a player to set things up and if you want rewards, to provide the rewards....this is how it should be.