Hence that default behaviour. I'm not really sure if this default behaviour is user-friendly or system-hostile.

user-friendly and system-hostile are generally synonyms

When this sort of thing happened back when I was serving my time at the hell desk, I used to point the 'student and professor' (in my case it was 'intern and clueless project manager') to the magical GNU compiler collection and Eclipse

So TRWTF is that MS failed to enumerate the reason(s) why a reboot was necessary, prompting our speculation. The correct way to handle this sort of thing is to identify components that will have to be written to in order to complete the installation prior to starting installation, and inform the user of any conflicts related to write-locks etc. so that he/she may try to resolve them; at the very least this will promote educated users who can solve their own problems or at least have detailed error data for tech support if/when they call -- see the Linux community for real life examples :) Also, if we're bashing Microsoft I think the fact that they still include spaces in default executable paths is a worthier object of ridicule...