Is it bad practice to use legacy body movers?

Old body movers have legacy status and not deprecated. That means they are not going away anytime soon, but will no longer be updated. Huge amount of games will stop working if they decided to remove them. Some of those games (especially racing) would be very difficult to migrate to new system. That being said you should really try to use new ones. Creating attachments makes sense, when you want more precision. I know they have a step learinig curve, but once you get the grasp, they are quite useful. I especially like MaxVelocity and RigidityEnabled. You can achieve constant speed easily.