The META Description Tag was designed to be an integral part of page development and document identification.
When dealing with the <head></head> section of your document, there are three pieces of metadata that I make mandatory on most pages that I develop; charset, description and keywords. I'll talk about the META Keywords Tag in another topic (don't obsess over this one, but don't ignore it either).
The META Description Tag is still an important factor in the overall equation of page development. Everything you do to that page will have an influence on something. You know, that "for every action there is a reaction" type thing.
Over the past few years, I've experimented with different strategies for the META Description Tag. I've settled on one which I feel has been of the most benefit. I will typically build the META Description utilizing content from the first one or two paragraphs on the page.
An interesting thing has occurred by formatting my META Descriptions in the above manner. Google will actually display full snippets of the META Description Tag when the proper keyword phrase or keyword sequence is queried. Yes, I've confirmed this by utilizing slight variations in the description as opposed to cutting and pasting an exact copy from the page. In many instances, the exact META Description is displayed in the SERPs (Search Engine Results Pages).
When developing your pages, utilize the areas you have available to you for describing that page to various resources. As far as I know, the META Description Tag is considered in the overall equation of page relevancy. Use it effectively and follow the suggested standards for formatting.
In addition to the above, think about those who are going to be viewing your pages from a technical standpoint (viewing source). This would include Directory Editors, Webmasters and other Internet Marketing types.
When I develop resource areas for my clients, the first thing I do is build a title and description for that resource. The first place I look is at the <head></head> section of the page I'm linking to. If the web designer/developer have not made proper use of metadata, I've got to go the extra step and build a description from the visible content I see on the page which is somewhat of an inconvenience.
Another effect of utilizing the META Description Tag is from a spidering standpoint. Many of the automated programs for building directories and/or resource areas will spider the title and description. Since the user is usually presented with that information in the SERPs, a well written META Description presents visual appeal!