That's a fantastic question. I wish that I had a fantastic answer, but I'll try anyway
If you put the money in the Roth, then I do believe that there are situations where you can take the money out pre-retirement without much ado. College is one time, and I think that home purchasing is another. (Someone correct me if I'm wrong.)
But, once you take it out, you can't put it back in any faster than otherwise allowed. It depends on how close you are to retirement whether this would be a big deal or not. If you're 25, you're forfeiting potentially a lot of tax-free compounding. If you're close to retirement, not quite so much. If you're taking out even as little as $20k that will take several years to put back in.
I guess just run the numbers to see what the effects are.