Eh, at the end of the day. Total software sold is the only number anyone cares about. So the 360's killing on that front.
I mean the gamecube had a MONSTER attach rate. Why? Because mostly only Nintendo fanboys bought it.
Attach rates really just show how "hardcore" your audience is... which really is a bad thing, because it somewhat implies that you arn't hitting that casual/mainstream audience that just doesn't buy more then a few games a year. Which, while they don't contribute as much, they still count... and are what are going to sell the most consoles.