All valid transactions are equally legitimate

1. People will always be able to stuff arbitrary data into valid transactions.
2. There is no test possible for transaction legitimacy.
3. "Spam" is a subjective term.
4. Bitcoin answers subjective questions with markets.
5. The fee market is the best way to determine which valid transactions end up in blocks.

The sender of each transaction should decide for themselves how much of a priority it has

If we have to decide which transactions should and which transactions shouldn't get into a block because we have limited space, there's really only two ways to do that. One is to specify in the software which transactions are good transactions and which are bad transactions -- what I euphemistically call a dictatorship of developers. Think about it. If developers get to decide which is a valid transaction that should be prioritized versus which transaction is wasteful or spam or for an irrelevant application that they don't particularly need, we just created a dictatorship of developers. They decide which applications are now allowed on the network simply by controlling that function. There is only one way to correctly price the scarce resource of blockspace in a way that doesn't give any one control -- and that is a market. So fees operate as a market, in order to give the freedom of the sender of each transaction to decide for themselves how important the transaction is and how much of a priority it has in their perspective.