This might have been discussed before...if it has, ignore me. We saw Kakuzu's skin get darker, right before he punched the shit out of door to get to the monks, right? On the same vein, when Rock Lee fought Gaara, when he opened the gates, his skin got darker as well, and of course, he got stronger/faster. Could it be that Kakuzu somehow is able to selectivley open gates, or is a master of his own gates, or has his 8 gates permanently open or something? Just a theory.