Most of ours are in use - and if not - we almost always point them towards another related site to pick up the extra incidental traffic.
Also, I always think it's worth keeping an eye out for expired or unwanted domains - eg we only picked up webhosts.co.uk in November last year after a chance whois on it and now around 40-50% of all traffic comes via it with no advertising at all (only just recently changed my sig to it here and on some other boards so can't really count any hits from that).
Cheers