Also consider how the battle would've turned out had Gaara went psycho when fighting Rock Lee. Lee would've probably MURDERED him. Naruto isn't that physically strong, and just when he was punching Gaara, put some ugly dents in his image. But Rock Lee would've blown a hole through Gaara in that form. The show wouldn't do that though, 'cause they wanted to teach Gaara a lesson, and have him (probably) join the good guys later on.