Personally if a directory I go to asks for a recp I leave. I don't want to go to the bother of setting it up on my site. A recp (unless as a few has suggested gets you a sponsored type of listing at the top) is a waste of time. Asking for a compulsary link or no lisitng gives me the impression of desperation.
I do likewise. There's no shortage of directories who will link to your site without one, if you know where to look.
There's also the question of how carefully you check the link. Do you look for robots.txt exclusion, nofollows, etc? And is the recip page even linked to from the main site, or indeed anywhere? How often do you check back?
Not getting shafted on reciprocals involves a lot of policing, even though it may cut down on spam submissions. Your directory is also likely to get more links in from various directory lists if you don't ask for a recip.