It depends on the type of forum. I think for a promotion forum it is a really good feature as it encourages people to post to get services. Then it makes them stay active to a limit to get more cash to keep requesting the services. If it was just post count based. Once they hit the post count, they can just keep coming back every month and requesting and not posting elsewhere.
General chat forums just have it to increase post count really and they don't really have a valid use for it for my opinion.
As Fowler says, virtual cash works very well on promotion forums. However, you need to be careful with it. If members are posting for cash and not because they are interested in the topic, this can bring down the quality of posts on your forum.
Good point Christmaßtatic and that's a useful addition that a lot of forum cash mods have. It can help to improve quality of posts. However, unfortunately longer posts doesn't always mean better quality posts.