First time I read Garoth's post, I thought he said portables shouldn't have their own screen.
But I think I see what he means: like the AV out option on PSP-2000/3000, it should be a standard option for consoles.
Yes, multiple controllers is cheaper than multiple systems. And also only needing to buy one copy. (yeah, I'm aware of single-pak Game Link on GBA, Download Play. But don't they usually have to cut features to make them work (RAM limitations, transfer speed, etc.), compared to using a separate copy per player?