The main value I'd get from such a button is bring able to mark things as helpful for my own easy reference later. The Wikis are great, but there are some specific threads/posts that may not go into them but may be useful for me in the future, and I guess not all of FT is as well served as the BA board in this regard. Mainly for this reason I voted in favour.
I'm ambivalent about the subject generally apart from that, not against it but have no particular desire to see counters of how many posts I have found helpful, or had found helpful by others (though hope some of my posts are of help to others).