I very much agree that the description can be a free ad, but note that Google does not use the description for ranking.
Care should be given to crafting a page-specific description which will contain text that includes the most likely-to-rank queries, so that the description you write will be returned in the serps. Otherwise, Google will pull descriptions from a variety of sources, which might include text on your page.
When matching text is not found in your description or on your page, Google pulls text from the dmoz.org description for the page, should such a listing exist.
See this discussion for details on how to use the NOODP meta tag to exclude DMOZ/ODP titles and descriptions from displaying in Google serps.... Google Supports NOODP Tag http://www.webmasterworld.com/google/3005968.htm
NOODP is now supported by all of the major search engines.
Do i still need to write down meta description for each?
I make an effort to do this, but on a large data-base driven site it's not always possible. You may want to experiment to see whether you prefer the snippets Google generates to your own auto-generated snippets. If you have a fairly text rich pages, it's often better to leave this to Google. Don't create one description for the entire site, though.