One theory (mine) as to why string through was conceived - Bear in mind the Telecaster was (I think) the first type of guitar/stringed production instrument to have this option. The reason why, I think, was probably to keep the bridge in place as Leo's first bridge designs were simple affairs, meant only to raise the strings high enough over the pickups and neck for proper playing action. His very first guitar design - The Log - in the late 1940's, didn't even have a bridge so the strings had to come through the body. String through for the Tele was probably a nod to The Log design and carried over into the Tele. This rings true for the first Precision bass designs that are also string through, which definitely held the bridge in place. It seems to me the bridge on the first P bass design was an afterthought - a very small, light bridge held in by only a few screws to keep it from moving around. He was tweaking bass bridge design and would obviously keep tweaking it until the "new" P bass came out. If top loaded the string tension would've pulled that bridge right out. I've had top loading bridges pop out of cheap basses before. My personal preference is if I have a bass with a string through option, I'm using it, because, why not?