LMA doesn't grab many rebounds because he helps a ton on defense, he's basically the anti Kevin Love/Carlos Boozer. In the end, LMA contributes a ton on defense and is really good defensively despite his lack of rebounding since contesting shots and covering hard on pick-and-roll matters more than rebounding if you have other rebounders.
I've watched this team enough to know that LaMarcus is their best perimeter defender next to Batum. That doesn't really speak to LaMarcus' wing-defensive skills (excellent for a big guy, I guess) as much as it does for the guards' defensive executions many times.