I've been riding and racing bikes since the early 70's and I have never seen any dropout break other then in an accident. So even though the strength thing makes sense, and yes it does make sense, I've never seen a dropout break even on heavy loaded touring bikes or when clydesdales were riding on horizontal dropouts. Maybe that problem may have happened to cheap WallyWorld or Sears bikes etc but not on the better bikes. In fact most bikes I ever saw fail was when strong clydesdale types put too much torque on the frame then the frame would seperate, but most of those breaks happened to older AL frames and the frame snapped usually in the BB area.