I think that when you do need code in your build that code should be first-class. Which means it should have test coverage, a release process, follow your coding standards and all the rest of it. The maven plugin model encourages that.

And maybe this is making a virtue of necessity, but I find the overhead of creating a plugin stops people from putting random "different compiler arguments on a Wednesday" conditionals in every build, which is all too common if you give them immediate access to a turing-complete language.