Sometimes I get absorbed in a side project, and I'm being creative and having fun. Why put a brake on it? I think that some of the best things I've done (professionally) were when I was in a "being completely absorbed in a project"-mindsets.
Sometimes I don't works on side-projects for months at a time. I just don't feel like it and I'm doing other stuff. That's okay, too. No pressure.
If you're always being absorbed by side projects then there might be a problem, but as long as there's is an ebb and flow to these things I don't really see the problem.