Oh, I think it's definitely going to have an option asking you if you want to download "Demo X" or not. Nintendo would be stupid to force that upon you. Especailly when talking about patches. How is Nintendo going to know you own the game in the first place?
Instead, I think once you turn on your console (which boots up really quick btw), it'll go to a menu showing you that you have this many (#) downloads available, this many (#) messages unread, someone left you a gift in AC etc. etc.
Basically, it'll become your little hub world for all things gaming (especially with the inclusion of Opera). And if you want to download something, I'm sure it will have an option to this effect "Download next time I power off the console." So you can check off a bunch then pass out only to wake up to new games and such.
But, I see your point. There really is no major reason to have your console on 24 hours a day. Unless........... unless there is something like that AC example Nintendo always mentions. Where I can roam your village while your asleep. Something like that would NEED your console on in order to work.
However, that idea isn't even that intriguing (unless of course, AC is dope as sh!t). Still, the possibilities are out there, we just aren't creative enough to see them.
/rant