The libraries aren't good for PLT scheme, at least for web development. For example, want to parse an RSS feed? Compare http://www.rubyrss.com/ to using HTML Prag (http://www.neilvandyke.org/htmlprag/).

Want to do billing? openid? you are out of luck. But having said that I'm experimenting with a site in PLT Scheme (and I know a couple that use Kawa as well).