So what if PS3 supports 7 boomerangs the games still have to support it. I remember Worms on the dreamcast forced you to pass the controller around even if you were playing with only 2 people. Sure the Dreamcast supported 4 controllers, and I had 4 controllers but the game didn't support it, so it was useless.
Do you really think the rich greedy bastards at EA are going to include 5+ player support in the new Madden if all but 1 console only has 4 controllers? there's no way in hell they're going to go the extra mile for that last platform to give you 5+ player support. Especially since there's no competition to worry about. If there was more than one platform... maybe.
Besides even sports and party games would get way too crowded at 5+ players... think of it this way... in sports games the camera would have to be zoomed further out loosing detail. In party games you'd get the same thing, in addition to certain game types that would have to be dropped with those extra players because the game simply wouldn't be able to handle it.
In the end if you can afford the extra $400 in controllers for the occasional 5+ player game you can probably afford another console and TV to do system link.
...and if you're complaint is that they games you like don't support system link... then you have to ask yourself if those same lazy developers would bother to go the extra mile in supporting 5+ controllers?