Incentive to Defend
You dont really want to encourage defense so much.
Some people may say lolwtfyouon but keeping up the momentum of WvW is what keeps it fun and entertaining with a steady stream of players. When this slows down too much – be it from fortified keeps, siege humping 10 ACs or having a huge map layed out like a kittening maze – WvW activities grind to a halt. It runs out of steam.
We need that that rapid pace to encourage aggressive attacks. Maintaining PPT for the server is all the reward we need for defending (and we still get lots of contribution for it, since defending often involves killing attackers).
The main incentive I want to give is to go out and kill attacking players, so this will result in the defender attacking if possible instead of just stalling, which sounds like a good thing to me.
The way you describe it offense is the only valid way to play WvW, but in my eyes defending should also be encouraged as this is what ultimately leads to fights, otherwise you end up with an EotM-like karma train.