>> Increasing the display area, restricting user navigation, giving the feel of desktop applications,

Fair enough, but one doesn't need to disable back functionality for this, you might want a popup, or just provide fullscreen browsing instructions. The backbutton's actual functionality is irrrelevant here, since the intention is, I guess, to remove the toolbar. You still can't stop the user pressing backspace, or having some registered shortcut that does 'back', or even showing the toolbar; but, I guess in these cases, that wouldn't matter since the reason that the button can't be seen is purely superficial.

>> naive guarding against the infamous back button syndrome when using AJAX etc.

It's a naive and unreliable means for restricting or controlling any part of an underlying application. Anything the user couldn't do without a backbutton, they could do by some other means; trying to hack out the backbutton because it messes with some assumption in the application is addressing the wrong part of the problem.