My goal isn't to lock everything down
Sure you shouldn't lock down all existing plugins, but please do this for all plugins that have bindings to the various build packagings. The packaging "maven-plugin" uses maven-plugin-plugin and hence should be locked down. Likewise, maven-clean-plugin is automatically bound and should have a default version in the super POM for the novice users to get build reproducibility.
What's the benefit to locking down clean?
You could have also questioned "What's the benefit to locking down build plugin XYZ?" and the answer would be the same: a reproducible build. Reproducibility also includes the little aspect of stability. If clean-plugin-X works but clean-plugin-Y fails the build, this is not reproducible. Just to be clear: I don't want to upset anybody but please let's be honest, the implication "version X worked so version Y will work, too" is just utopie. This in mind, please consider that clean is executed by the release-plugin.