I don't think they're worth it, to be honest. I mean, let's be realistic. How much traffic do you think a topsite alone can reach by itself? I can't see people visiting topsites daily.
I've added my forums on topsites before, I even made one for my old forum that I turned into an archive forum. ( I was actually surprised when someone actually added their site!)
I say it's worth a try, but I wouldn't put too much effort into it.
"You never know" after all.
I'm neutral on the subject on bringing traffic from the topsites themselves. However, if the topsite has proper SEO, you might be able to get backlinks and google indexing and ranking from the topsite link. That in turn might rank you higher on search engines and get you found for search engine traffic, which is important on one end. But of course, you wouldn't have 50 topsite links on your site/forum page just for the hopes of getting this result. If even what I say hold merit. I've never tried it before. I'm just going by what I've heard about 1-1/2 - 2 years ago. So bear with me...
If the topsite has a high pagerank and is relevant to your forum's genre you might get some benefit, because a search someone does for your kind of forum might pull up that topsite and see your site listed.