It also sets up an interesting DOS attack where potentially a miner with a large bank of computers could charge very high transaction fees, slowing down or even potentially stopping trades. I guess the way to stop that would be to get a bunch of computers for yourself and hope you can squeeze in a successful hash at least for yourself to get your transactions put into the network without a fee (presuming you can be selective on the fees for your friends and own addresses).
Well, you can do all sorts of stuff if you have >50% of network CPU power.