I think it was already mentioned at some point, but I do think uninstalling the game on steam will revoke the authorization and get you one slot back. So for example, I mainly have 1 gaming machine I play on always. Say I need to reformat my hard drive. I just uninstall the game on steam and I get the slot back (from 4 out of 5 to 5/5 slots available). I could be wrong on this so sorry in advance if I am. I do agree though that securom is unneeded and a pita.
