I'm sure you will all agree that a Meta Description is your tag line and is responsible for your website's final promotion as searchers decide which URL to choose from in the SERPs. Even Google says that 'the quality of your snippet — the short text preview we display for each web result — can have a direct impact on the chances of your site being clicked'. If you cannot decide this yourself, how can you properly market your website? Obviously Google will check the Meta Description first, but what if it does't contain the keywords chosen - it will pick a bit of random paragraph??????
If you don't use a meta description tag, Google will pick what it feels is most relevant. This may not be the best snippet for your conversion rate though Even if you do set a meta description, Google can still make its own mind up, and override your choice! I myself have seen different snippets shown for the same page, depending on what keyterm is used to find that page... Test it yourself? Find a page that you know ranks for 2 or more different terms, and see if the same snippet shows for every term? Sometimes it will, sometimes it won't - it depends if Google thinks it has 'seen' a more relevant bit of text on your page, that it prefers to use as your snippet! You can also add a tag to tell Google not to use the open directory entry your site may have as a source of meta data ( http://www.mattcutts.com/blog/google-supports-meta-noodp-tag/ )
I think it is just important that the main keys you want your page to rank are mentioned in the meta description tag. Also use different meta description tags for the different pages.
Even if you have a meta description, there is no guarantee that Google will show it as the snippet for your URL in the SERPs. It helps to understand how Google chooses search snippets. Optimizing your meta description is very important. Though its not used by Google as a ranking factor, a well optimized meta description can drastically increase your click-thru-rates. Each page should have its own custom meta description containing the targeted keywords from the title of the page. It should accurately depict what the user can expect to find should they click through to your page. It should contain a clear call to action as well.
Think like this each page of a site contains the same starting of the title but the last part of the title is changed according to the page content. How do that help? For description the same tactics is use. will that do any problem?
Google selects the most relevant content from your page and displays them in the SERP. Also, meta description in NOT your tagline. It just needs to a short description of the page in 1-2 sentences. A tagline is a phrase of usually 4-5 words. Very true...