I always figured it was simply for mileage anymore now. Used to be because they could build the front driveshaft cheaper, smaler, and probably didn't bother to balance it very well, and those low pinion axles didn't exactly help the u-joints out. My friends J-10 (damn I use that truck as an example too often) front driveshaft is about a big around as a half dollar. It's little more than an axle shaft. He left the hubs locked all winter, ended up roasting the splines on the slip yoke and now it wobbles like crazy.
I have to imagine that if it caught this water separator just driving along, it would have caught it anyways? I could also bet he smacked it on something in the field and the highway speeds tore it the rest of the way apart.
Besides, a ton of vehicles now have full time cartridge style bearings up front, those don't seem to have issues?