For anyone who doesn't possess the body of a Uchiha - using an implanted Sharingan is going to be a double-edged sword, and according to Itachi will never function entirely as it should.
Obviously, it's had it's uses and it's saved his (and everyone elses') ass more than once. But how many times has it left his weak and vulnerable or out of action? Weeks of recovery - long drawn out battles like this war are really against him.
I wonder what level his chakra is at without it... always considered his 'stamina' to be average, but I guess without the constant drainage his general reserves should be somewhat impressive.
... though compared to everyone else there, I'm not sure how he measures up.