If I grab a window with Alt+click and move it, allow me to move the top of the
window off the top of the screen.  Here's why:

1) Normal users that don't know Alt+click won't ever lose their titlebars,
because they will always move their windows by dragging the titlebars.  
2) Advanced users that DO know Alt+click want this functionality.
3) To solve the problem where a user mistakenly holds Alt and clicks a window,
and it moves off the top of the screen (I'll bet that this is the reason that
this functionality is disabled): make it a hidden preference, and make the
preference disabled by default.

Other information: