Back to the OP - is there a difference between a bad player or a good player that consistently puts in bad performances?
A bad player can be relied upon to produce a certain level of performance, even if that level isn't too high. You can organise your team to cover up his weaknesses, you can only deploy him in certain situations and so on, you've got a whole host of options.
A good player playing badly is worse, as you can't build anything on him, no consistency to his game, not even in his mistakes, there's literally fuck all you can do.
And that's before we get into how it affects the dressing room.
So for practical reasons I'd much rather have a shit player playing his heart out than a good player who can't be arsed.