Ahh I hope this thread is still alive.. My friends and I have argued about this since WE7 when we started playing the game.. He thinks it does not matter but I think it definitely matters quite a bit. Think about this when you have the ball, you are only controlling the one player, and if your team mates are all on 3 stars (as opposed to 6, I can't imagine anyone would wanna vs their friends on 1 star..), they will not be as active in finding spaces to run into, and they will not position themselves as well for a potential pass.
As for the defending, remember most of the time you are only controlling the one player, unless you're one of those guys who defend with the R1, x and [] down permanently (If that's the case you should be playing fifa). So there are 10 more defenders out there trying to position themselves with a lower A.I. They won't read the plays as well, and by the time you switch to them it might be too late as your friend's striker might have found a gap to run through already..
Another way of looking at it is if you've played soccer on the field, 3 stars would be like playing alongside some high school kids who are reasonable at what they do, but they are no where near as good as the professional players which are on 6 stars.. So I believe it's much more enjoyable to play with better players since that's what I play against the computer on... I can predict my team mates moves a bit better..
I hope I made sense and I'm interested in people who thinks I'm wrong about this and support my friend's view in that it makes no difference.