Or until you need to guarentee the version of the library you're bundling

Or you want to run on something other than a MAC-in-crap, like Linux.

RVM has the same problems.

In my opinion, one is for play, and one is for work. If you just want to fuck around and spew code in one long controller that looks like spaghetti, and you don't want to worry about reproducability or quality guarantees or modularity then Ruby build tools are fine to use.

It's incredibly frustrating to hear about people complaining about how tool X doesn't do task Y. They could just STFU and write the plugin.

There is really only one type of plugin that wouldn't work well in Maven, and it's when for some reason you're task doesn't fit into it's "life cycle" pattern. For example, you want something running in the background when you edit SASS, LESS, TypeScript files and you want some processor to just run and deploy in the background continuously. In this scenario, you could keep the Mojo API, but you would have to lose the Maven Framework.

Each tool for it's job.