I can understand where you are coming from Chelsea, I guess my methods are built more on my own past experience. All leather is going to stretch. I have been asked numerous times, if I can "fix" other vendors leads that are attached in the method that you are referring to. The leather stretches and the point that the two strands go over the hardware becomes weaker until it finally breaks. My cores do not tie back onto themselves, but I do think they provide some strength to the lead. But, as I mentioned in a previous post, I think a lot of the strength also comes from how well the back braiding is done. When I first started braiding (8+ years ago), I actually did a 4 strand flat braid over the hardware connection to cover the core tied to the snap. But, over time, the core started showing more and more. If I'm going to charge $100+ for a lead, I don't want to see the core. I can honestly say (knock on wood) that I've never had a lead break, at the snap or anywhere else. (Knock on wood again!!!)
That being said, I have had to replace a couple of small, broken snaps on leads before. When I pull apart the back braiding to remove the broken snap, the core is still completely through the round braiding portion. However, if my leads were not rolled and stretched properly before back braiding, the core would be pulled out of the ends after being used for a little while. I highly recommend Gail Hought's books for any braiders, and this is the method she uses for all of her braided products as well.
Shannon