How big of a problem is a microbranch in Subversion? Greg pointed out that they aren't stored efficiently, but if you make a modification in a branch and then integrate it into the trunk, it only doubles the storage of diffs, no more (at least if I read his comment correctly) -- that doesn't seem so bad. I know I underuse short-lived branches, but Subversion doesn't disallow them; you can delete a branch as soon as you are done with it. The only real issue is that its merging is rather poor, so it discourages branching.
That said, I do see the usefulness of "spontaneous branching" as Mark termed it. There is a certain oddity to the special place the checkout has in Subversion, where in distributed systems there isn't anything quite like a checkout, and there's an opportunity to be somewhat more cohesive as a result.