We see this in all the pvp games. Farmers will figure out a way to work the system and ruin things for the rest of us.
Increase he reward for taking a keep suddenly no one defends. Give some xp to some one who died in the fight so they will be willing to risk charging, just stay dead.
This is exactly the point of keeping PvP with the least amount of "unnecessary" external rewards that don't pertain to PvP. ... no need to make lots of gold in PvP, it has it's own coin system, so keeps gold farmers out, no need for major XP as PvP should be about the campaign and end game content (RvR aspect at least), I'm not really even a fan of small quests in PvP... anything that distracts PUGs and guilds alike from having the single goal of getting those keeps, crowing an emp for the campaign buffs, getting the scrolls... and of course smiting our unworthy enemies is defiantly important, but not as much as the others.
IMO if it does not lend to PvP enhancement, keep it out. Make the PvP lines level faster in campaigns vs PvE, make xp bonus every soo many ranks earned in the alliance... they should all be focused on PvP rewards. Keep PvP about the big goal, and keeps it more fun. Every change that adds rewards not for PvP gives non-PvPers reason to exploit it and ruin our PvP as a group.