Automatic cleaning of the sandbox is not wiki-like. It might be of help for super high traffic wikis, but even there archiving would be better. There are people out in wikilandia who keep their first wiki edit for memory, that would be gone if cleaned. Just as useless as the chat page on moon-edit that cleans when the last one leaves. The sandbox even gets used for developing contents together. Why not, it's just an ordinary wiki-page. The first one people use very often.
I made two sand-wikis on hives. We'd need one here too ;)