Phone manufacturer (and your phones original software developers) have more access to your phone when you do not make this update. These updates have been made to give them less access (as well as just to make phone work better because internet sites have changed and in the software are old packages which may give access to the phone more easily if the software is not updated). But I think you have now enough information why you should update. Otherwise it would be better not to use N900 if security and cotrol over your device is your goal.