Hi. I would say there are only pro's of wearing a seatbelt.
Safety is of foremost importance when it has already witnessed and proved that many deaths happen on account of automobile accidents.
Why some states enforce and some dont? Well there are few angles to it, Commercial as well as statutory.
Commercial angle reflects the insistence of few safety equipment manufacturers who enjoy a authoritative position and are in a way to influence decisions to market their products. Stautory requirements reflect the Govt's need to safeguard its people interest thereby demonstrating a greater sense of accountability and belongingness.