It really depends on the page and the website. But, as a rule, I think it's always smart to write a meta-description to gain the trust and attention of your potential visitors. As you probably know, the meta-description usually shows up under the Title and URL in a Google result. This bit of text could potentially set you apart from your competition.
It's true that Google will grab a bit of text and use that for a description of the page by default, but Google is a robot and probably isn't going to grab the most important text in the article. It will simply grab some text with the related keywords usually. It certainly isn't going to put together a catchy sentence / summary for searchers to read.
To make this as clear as possible, let's take a search for "cheap headphones":
We have the #1 site that uses a meta-description -
"Cheap headphones and discount headphones, Get information on all types of headphones and find the secret to getting the best deals and what website sells them for the best price."
Then we have the #2 site that is letting Google choose its meta-description text. It looks like this:
"20 results – Our top picks for the best cheap headphones around in 2010.
| $19.95 | sugg. retail price: $22.95 | Kidz Gear Headphones | Excellent ... |
| $19.99 | sugg. retail price: $19.99 | Koss KSC 75 |
Surprising high ..."
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These aren't great examples because the #1 site has a description meta-tag that is too long. But I think it gets the point across. It's a lot nicer to read something that is concise and grabs the searcher's attention than to hope Google is going to pull the best information for the user. Human wins vs. Google this time. (for now).