If there were a consent of a majority of miners not to include transactions this might become a problem. As long as there are enough miners trying to include the transactions, I don't take this as a problem. Basically it's a kind of > 50% attack you are talking about. And especially with PoS it is an economical problem to place a > 50% attack, because successfully executing such an attack renders your coins worthless without having the chance to spend them soon (they need to mature for 520 blocks in case of Peercoin and having them successfully used for creating a PoS block before the can be spent).

You say there is no incentive to include transactions. I say the incentive is to keep the network sane. If you want to successfully exclude transactions you risk the whole concept. If you try to do it successfully, you need to have a big share of the coins. You can do it, but you shoot yourself in the foot. I tried to do some math here. Maybe you find that interesting: https://bitcointalk.org/index.php?topic=326216.msg3526904#msg3526904

Where is the incentive not to include transactions?
Can you please explain the advantage of not including transactions into a block?
...maybe I still miss the point...