[haldean]

The point about links is that other people can profit by posting links on your site. Before Ad(Words|Sense), you could freely allow people to post links in comments (without having to worry about nofollow or anything) because people would only profit from those links if what they linked to was relevant/interesting. Now just the act of following a link can mean a few pennies in someone's pocket, so allowing links to be posted to your site is a quick way to get seriously astroturfed.

[kamjam]

Right, fair enough. Stackoverflow used to add in a nofollow to all links posted on their site. I think they have removed it now for users of a certain rep.

If your site is small, then just moderate the comments yourself. If there are not that many then it will not take much time!