I am contractor, most of the time working on corporate PC. I use dvorak and it is already a huge PITA to explain to yet another corporate drone I need to be able to set up my own keyboard layout. Windows does not have Dvorak layout with polish accents so I have to either use my own driver or, more likely, write without polish accents.

Another problem is that Windows can have different keyboard layout when logging in and different when unlocking the machine which might cause additional login failures. Some applications try to be smart (which rarely works) and control keyboard layout. At one bank the remote desktop app would switch keyboard layout periodically (like every 5 minutes) depending on my locale settings.

So this is with Dvorak, which is popular enough. Try with a completely unknown layout and you are pretty much screwed if you ever need to be able to use computer that is not your own.