Better than keeping them idle. Even for free you lose nothing - domain just gets more popular and links. A friend of mine had his site blocked by his proxy by mistake - took 2 months to clear up. In that time he use a spare domain I had - then went back to his domain when the problem was resolved.
My domain is now back - and gets over 100 visitors daily because the site it used to replace was a huge forum.
Just mind they take good care of it while it's "rented" - if used for spam it can be fined. and you might also be held resoncible for anything the peopl who rent your domain do.
Make sure that you have a lawyer write up a contract wherin they agree to be held legally responsible for all the content on the site in the event that you are sued. I forget the correct legalise for it, but it is a fairly common type of clause
Also, I'd recommend getting 6 months rent up front. And the one thing that I wish I'd done last time was to put provisions in the contract for late fees if they stop paying you..
(indemnity clauses aren't enough, though, as they might have insufficient assets to indemnify you from the risk; you'd want a clause whereby you can block content from appearing on the site that you deem, in good faith, to be illegal, etc.)