I have to agree that since there is the TX fees, which can be even as much as 1BTC or more, it makes no sense to mine empty blocks
Seems so stupid to skip some of the reward. When the block halving happens, the TX fees are going to be even bigger part of the total reward.
U'll find that most of the empty blocks are mined with little time gap from the previous block. The reason is, they just found a block before setting up the unconfirmed Tx list and hence published.