1script - 9:55 pm on Jun 3, 2013 (gmt 0)
I had a similar approach only I've taken the beginning sentences of the UGC snippets because there would be no good way to find what's valid or not. I have also done it for years (at least for the last 10) and I'm not going to change it because if also works well with Bing and I value Bing traffic very much. It may not be exactly the best description but at least it's not completely built by a robot either, which would have happened if you've removed the meta description and let Google do it for you.
I used to combine (based on text length and word usage and simple text analysis) sentences that could be considered valid, with other sentences from later down in the thread replies (if those in the initial post were too short). This worked well for many many years.