Even a plain leather strap will stretch, which is also a problem.

I did a lot of internal debating on this one, and here is why I dropped the money on the toshi doubles: Doubles are stronger, safer, and only like $30 more than the singles, so it was kind of a no-brainer between the two. They sandwich a layer of nylon between two layers of suede to prevent stretching. Notice I said suede and not leather. Because the suede is, well, suede-y, the serrated edge of the buckle doesn't so much cut into it, as it would with leather or nylon, as sink into it, thus helping to prevent an extremely weak spot in the strap where failure is likely in the event of an emergency.

I could be way off-base on any of this but it's basically the conclusion I came to as I researched the subject.