TheOptimizationIdiot - 7:05 pm on Apr 23, 2013 (gmt 0)
For example on user generated pages like forum posts I just use the sites primary meta description since the user is not asked to enter one when they create a forum topic.
I'd leave it off personally.
It's not an independent scoring factor as far as I've seen in testing or heard from testing at all recently plus if your "primary" description doesn't "fit" the topic of the results the page is returned for -- which I'm guessing it won't in most cases -- then either it's going to:
(best case) be changed by Google anyway.
(okay case) not be changed by Google and not generate a "more compelling reason to click" -- not hurting but not helping.
(worst case) not be changed by Google not match the title in the SERPs or the query and generate fewer clicks than if Google used some of the text from the page -- could possibly hurt if you rely on Google and the algo doesn't do what you need it to for more clicks.