Yup, ditto. Ended up writing a companion novel for the main series I'm working on for that same reason... and it helped me flesh out the main series.
Probably helps that I would highly consider self-publishing if I can't find a home for the series, so I don't see it as a loss to write the full series first.
I've never been one to think in series, but several weeks ago I wrote a tiny plot point for my current novel and it exploded into an idea for a companion novel that would be loosely related to the world I'm working in now. I have a feeling if I write that book then I might come up with another loosely-related book. This would be a situation where I would self-publish if I had to when it came down to it because I would (hopefully) have the sales boosts each time I released a new book.