I think it's a nice feature because you are able to buy items to improve your forum experience. On the other forum I frequent, you can buy username changes, a rainbow username, an icon which appears beside your username, a notice ad and much more. Just encourages members to earn more currency and contribute. 😀
I love forum currency. It allows people to work hard for username changes, and different site features. Plus in some sites it could be used as a competition tracker.
For certain types of forums it's needed, yeah. For the most part I think it's just a waste, unless people are going to be trading it or using it on a daily basis there really isn't a need for it. :/
Really, it depends on the "niche" of the forum. On our forum, users earn currency and then go to our shop and buy things from it. When they buy it, it adds the item to their mini-profile and profile to indicate they actually bought the item. 😀
I also don't think it's needed for every forum. I say it only really works for promotion forums like this one. If you just have a general discussion forum then I say the forum cash is not really needed.
One of our staff added one but we haven't really had much use for it yet. I think I'll use the ideas in this thread and try to implement some of them. ^^