How do you know that a majority of hashing power accepting no-fee transactions is going to be working with one plan to attack bitcoin, rather than preserving the value of their existing bitcoins (which would be worth less after such an attack) or preserving their bitcoin-based business or helping out because they like using bitcoin and would be inconvenienced if it lost credibility or something else? Or the same sorts of things with people paying fees.

Also, what if bitcoin clients included some code to use the processor and graphics card in such a way that they didn't use any more power (by avoiding stepping up the frequency or whatever) in order to hash? You could get a lot of hashing power that way, and some lucky people would randomly get some fees in their accounts.