I guess there are two issues. One would be the warranty. Now a cracked screen showing a personal screensaver might be what some folks would like to avoid. But in some cases I have read about - at the point the screen is broken, it might be impossible to UNdo anything anyway.
The other issue is that I think I read that a hack for an earlier kindle disabled updates. While the hacks do not seem to disable updates at this time, and folks are getting 3.1 with the hacks, once 3.1 is intalled you cannot remove the hacks. Folks are concerned that the potential NEXT firmware upgrade might be locked out by the hacks and so that way the ability to remove the hacks if that were to occur.