In fact, it benefits everyone. In general, accessible Web sites don't need to be designed to be very different. Authors can use HTML 2.0, HTML 3.0, HTML 3.2, HTML 4.0, and proprietary extensions and still maintain accessibility. The key is to use features so that they degrade gracefully in non-supporting browsers, and to avoid dependence on poorly supported features. In some cases, such as HTML tables, this can require some extra work and thought, but most new features in HTML 4.0 were designed with an eye on backwards compatibility.
Style sheets are an excellent example of a feature that degrades gracefully.
The proper use of style sheets can significantly enhance the presentation of a site while not deterring from the site's accessibility at all in non-supporting browsers.