Hi Mark,
I hear what you are saying, however my thoughts are the reverse. Here's my reasoning....
Take a sharp flint, striker or turnscrew and drop it into an inside-out bag. It is going to fall to the bottom, and rest on the turned leather bottom on either side of the seam. Not on the stitching.
With a flat bag, something dropped inside is funneled to the seam, and the stitching to a degree is what is stopping the item. So, in my opinion, I think a welt is more important in a flat bag than in inside out one.
Additionally, other than craftsmanship and beauty, what is the purpose of a welt in an inside-out bag? The same stitches (inside) on either side of the seam are still exposed the same amount to friction and wear from carried items whether there is a welt or not. There is simply another layer of leather inside the seam, in an area that is not getting wear. I guess on the outside of the bag the welt might prevent some wear, but those stitches are more up in a sheltered "channel" resulting from flipping the bag.
In other words, I think the things inside the bag have potential to cause seam failure more than outside influences. I could be wrong though. Especially if all someone carries is tow. :-)
I hope my explanation makes sense.
I just share this in the spirit of learning, not to cause any conflict.
God Bless, and best wishes, Marc