Certainly there is. Mining pools currently get 50 BTC for every block plus total transaction fees for that same block. For many of them transaction fees are the only revenue source. The minimum transaction fee effectively is a mining pool 'tax' for all network users. I understand the need to protect against DoS attacks. However, what is the point of protecting against an attack if there is no attack? If you're not flexible, always keeping your shield charged to the maximum, it is just a waste of resources!

The minimum transaction fee should be a dynamically defined variable by the system. Its value must depend on including DoS attacks (or other attacks) already commenced and in progress.