A snippet is a text excerpt from the returned result page that has all query terms bolded. The excerpt allows users to see the context in which search terms appear on a web page, before clicking on the result. Users are more likely to click on a search result if it has a corresponding snippet. If you wish to prevent Google from displaying snippets for your pages, use the following tag: <META NAME="GOOGLEBOT" CONTENT="NOSNIPPET"> Note: removing snippets also removes cached pages.
Some people feel that any meta tag other than the title tag and description are a waste of time. It seems to me that the no snippet tag is very under-rated. The description of your web page is often just as important in getting a surfers interest. So many people decide to let a robot decide what to use for a description on all of their pages. I find that most of the time i would rather decide what the description says for my webpages in the search results, even if it means the page will not be cached.
I would agree that the description is important. But I think your assumption that blocking the snippet will cause Google to replace it with the meta description. I don't think that is the case. I think they will just display the listing with no description at all.
Of course, I could be wrong. I haven't paid much attention to it in a very long time. If anyone is using the no snippet tag and getting their meta descriptions displayed instead, I'd love to hear about it.