A modern car will run into suspension wear/longevity problems long before the pillars care about how much weight you put on them. This hardware is tens, maybe a couple hundred pounds. Roofs can support thousands without deformation. The vehicle will suffer increased tire/suspension wear and "bad to the point of it being hard to justify not using a bigger vehicle" handling long before the structure is in any way affected. They stack cars on cars at many junkyards and the lower cars are just fine save some scratches and dents to the roof skin.