What's the protocol for dealing with Tumbleweed-like questions? (includes those with only unhelpful answers)
Should they be deleted? Left hanging?
This is assuming that it's not important enough to add a bounty on it.
Assuming these are your own questions you're talking about, it's really up to you.
Do you still need an answer? If no, you might consider deleting. If yes, you might consider adding more detail (which will "bump" the question and grab some attention).
The fact the the tumbleweed badge exists at all is evidence that Stack Overflow wants to find answers for old, unanswered questions.
They should be answered, of course!
If you don't have an answer, leave it. Maybe someone will come along eventually.
If you're the original author or you have enough reputation, and you now have more data but still not an answer, you can edit the question to add this new data. The question will go through the front page again and get another chance at attention.