There's not a whole lot that a simple body change can do that a chassis change cannot, if anything. Personally I've never seen anybody take extra bodies to the track because they feel one might be better than another in offroad because any difference is miniscule at best. It's a waste of time and money in my opinion, that's all.
As far as being negative goes, how was I doing that? All I did was ask a question because there are plenty of other Jammin X1 bodies available just on AMainHobbies' website, ready to paint and cut without having to screw around...