Meta descriptions are being used by google when you use site:domain.com but only for the simple purpose of displaying a description of a website when it's matched. It is not used in ranking.
For example. This is just a small scale example to illustrate my point.
http://www.google.com/search?hl=en&q...=Google+Search
I know that there is a review on my site which discusses norton-antivirus. So say if someone searched for "norton" on google, the result-text from google will not include the meta description but it'll include a snippet of the matched text from your content.
This really serves no benefit.