Auto Update works great if your PC is on broadband and guarenteed to be switched on at any given time of the day. ie if i set it to 8 am what if i switch on at 8:05 am thats nearly 24 hours before the next time it'll check. What if its set to midday - might go out in the morning and switch the PC off before it gets a chance. Same again with 6pm or any other time.
Good idea for people with Broadband who leave their PCs on 24 / 7 - not practical for the rest (who i suspect make up the majority of users)