The "request desktop site" doesn't work well with sites that dump you off into mobile sub-domains.
Frequently, you'll land on a site, it'll see you're on a mobile browser, and send you off to m.sitename.com or whatever. From that point on, no matter what browser ID is sent to it, it will always return a mobile site.
The trouble is that when you try to go back to
www.sitename.com, you again present a mobile ID, and thus get shipped off to m.sitename.com... etc.