When your programmatic build steps are isolated in plugins, then you can treat them like independent projects and apply your standard development practices like code review and unit tests to those plugins. Whereas when you stuff programmatic build steps into scripts that are bundled into existing projects, it's harder to make sure that your normal processes for assuring code quality get applied to those pieces of accessory code.
My standard development practices like code review and unit tests do not scale to review and test every dependency of every dependency of my projects. Even at company-wide scale.
I'm not saying that. I'm just saying that improved ability to apply such development practices is one benefit of using a plugin-style architecture for isolating the programmatic steps of your build pipeline. It's not perfect but in many ways it's still a significant improvement upon just allowing arbitrary code right in the pipeline definition.
