Paul,
I have to disagree with you because Microsoft don't allow you to activate over the internet because they no longer support the software. They tell you to activate over the phone but don't give a phone number to do this because they don't support the software any longer. I have to go onto a forum such as this to get the relevant phone number. Maybe I used the wrong wording when I said they were blocking, but they definately are not exactly bending over backwards to be helpfull.
As far as exceeding the number of CPUs on which your licence allows you to run the software goes, I have only ever ran it on two. The reason someone might need to install on another computer other than the original can be because you are upgrading, replacing a computer or many other reasons.
Never once have I been asked the reason for re-installing.