If it was any good, it would be showing the desktop scaling/layout/whatever instead of the mobile when 'show desktop version' is on.
It's also not always about being on a responsive sites, a lot of places still do a m.website.com redirect when visiting the main site from a mobile device, but with no backwards redirect when visiting the mobile site (which is almost certainly non-responsive, otherwise there'd be essentially no need for the m.* site) as a desktop client - so you hit 'show desktop version', the m.* page refreshes and you just get the mobile site with funny scaling.
Request mobile site is really only applicable to the small amount of websites that serve a m.domain.com.
Now, they could have an option to "request desktop SIZE" that would result in sites that are responsively built rendering in "desktop size" mode.