Meta descriptions definitely aren't a waste of bandwidth. One of the things I dislike about SEO is that it gives webmasters a sort of tunnel vision when they design their webpage. And I think recent attitudes towards meta tags are a perfect example of this.
From time to time, we have to remember that there are other considerations to be had besides SEO/marketing when building a website. Sure, meta keywords, meta descriptions, etc. have very little SEO value these days, but that's not the (sole) purpose of those fields. Search engines aren't the sole consumer of meta data, which is important for much more than helping you rank better on Google.
When you bookmark a page, many browsers automatically fetch the meta description or even keywords. This enables users to browse their bookmarks and find old links much more easily. The same thing happens when you post a link to Facebook, Twitter and other sites. Some screen readers also allow users to preview a page by reading out its meta description, giving the user a more detailed overview of a page than just the title.
The point is, like semantic structure, meta information is useful because it enables users and programmers to do more with a document. These could be uses that you never thought of or that haven't even been invented yet (e.g. a wikipedia bot can pull meta descriptions from a webpage to add descriptions to external links; a browser history search engine can build faster indexes from each page's meta description). Not including meta data just because it doesn't improve your search ranking is really shortsighted, not the type of user-centric thinking that webmasters ought to be using.