http://image.bayimg.com/4e6822913d1c31c0a756bca4ca4188e35d0443c8.jpgAll users of Bitcoin benefit from a secure network and a tragedy of the commons type problem could occur, in the sense that Why should I pay a transaction fee for my transaction, when others could pay the higher fee to incentivise miners? This problem clearly occurs for the second "reason" and not the first.
From the miners' point of view, they want to profit from mining, so they won't include txs without enough fee, and the fees makes the network safer, so the reasons for transaction fee are actually coinside of each other.
