So that is the reason. I've never understood why the lower-left corner thing worked on some XP machines and not others. Now I know, Thank you!
This trickery only works at the very edge of the screen, though. So if you click just a pixel below the start button (when it's moved upwards by the language bar) then nothing will happen.
I suspect that was once a rushed usability fix after noticing in testing that people apparently couldn't hit the button if they just moved their mouse right into the corner. And moving the pointer by a bit was perhaps deemed less likely to break anything than simulating a click on the button.
Or Microsoft just wasn't that good with testing back then (I think the language bar was introduced in Windows 2000, along with the problem), since most US users won't have multiple keyboard layouts or languages configured. Most users in other countries however do – at least I always got the US keyboard layout installed alongside with the German one (which leads to people being very confused about why their keyboard produces strange symbols because they happened to hit Alt+Shift by accident).
Uhm, I guess I got a little side-tracked here.