What would stop a well funded, bad actor from using a super computer to create blocks with only a few of it's own transactions in it? Someone that wanted to undermine bitcoin? I think it's a very bad idea for the network to not set some criteria regarding transaction inclusion into blocks.
If you have enough CPU power to dictate specifically which transactions into a block, we have bigger things to worry about (double spending).
There are plenty of validity checks on blocks and tx's in the client. It's just that fees are not one of those checks, permitting miners to choose their own policy.