If you own one it can increase visitors to your forum as people put a code on their forums that link to your topsite so when their members votes they see your topsite and can visit the forum and think "i might sign up here" and if you submit into one you can get quite a few visitors however you need to be near the top to get a lot of visitors.
Topsites are only beneficial if you can get enough people to vote. If you don't have enough people to keep your forum on top, then they lose many of their advantages.
As said before, if rated by pageviews, you can make it to the top without trying to beg your users into clicking a button. If the topsite has a decent pagerank, it could be very beneficial.
I remember back when topsites were really commonplace (they seemed to have faded from popularity over the years). They were large an ego stroke for webmasters.