I think sometimes the rear-loading track end is an affectation - and sometimes it is used because there may not be a good supply of quality horizontal rear dropouts for new production bikes.
When I ordered my Mercian Vincitore, I never specified which ends I wanted fitted. I did make it clear I would be riding it as a single speed or a fixed-gear (fixed-wheel if we must use older U.K. nomenclature). They then fitted Campagnolo track ends that were drilled and tapped for fender stays.
In hindsight, I would have been happier had I asked for, say, some old Cyclo ends, or some Stallard pattern dropouts. For that matter, I could have just asked for two LEFT Campagnolo 1010s - the lefts don't ever break, do they? Isn't it always the right/drive side ones?
Russ Fitzgerald
Greenwood, SC, USA
http://internaldetours.blogspot.com