Necessary ? No.
But I rarely use the button with the multitouch gestures.
The button is a great user 'safety' button though - any time you feel lost - press the button to get to home. It makes the device 'simpler', therefor more appealing to a wider audience, IMHO. Not sure about the benefits of removing it.
and also if Siri gets integrated eventually how would that work? Give another button double-duty? Also doing a reboot?
Eh, we'll see in 8 days.