How are they different as long as they are not spam? Good contributions are good contributions and help, regardless if there's a crappy t-shirt or not in exchange for them
It's the incentive structure. It's effectively Goodhart's Law in action: When a measure becomes a target, it ceases to be a good measure. In this case, the measure is "number of pull requests", and the result of making it a measure is that people created an insane number of PRs that simply adjusted white space in a README or slightly reworded phrases but kept the exact same meaning. Because they wanted the T shirt, because they felt that it validated them as a developer (ignoring that good contributions would have been a much better validation).
