Forum Moderators: phranque
Thinking as an average user (or trying to!), I'm fairly sure people would be more annoyed because they may need to close their main browser window. They may also be less likely to return. Ultimately 'disabling' or 'breaking' the back button is not what the users expect, and for most users familiar == good. However, if a user is pleased with a site, they are more likely to return, tell friends about it, etc etc.
As for legitimate reasons, I guess there may well be, although I have never had the need to do it. IMO if you are considering disabling the back button, do it on a page you expect closed.
It's a bit like finding that someone you don't know has disabled the reverse gear in your car and you have to get out and push the car backwards ;)