Don't know how much meta description affects SEO. But surely this isn't ideal situation.
Webmaster tools marks missing, but also too long meta description on my site. Also Google show only first 175 (I'm not sure with exact number) characters of your meta description in SERP.
I sometimes leave Google decide it's own meta description (by leaving meta description "tag" blank). But can't tell if this is good practice.
Some reading on this topic can be found on Google Support under "Site title and description" (not sure if I'm allowed to paste link here).