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.