Tim, I have no problem wearing a MB UL parka with the 58" wide in the Rev, and I'm 5' 10", 190 lbs, so not a small guy, but I guess I could see where someone larger might like the 64". As for me, it's about shaving ounces, so while I'd like the extra width, I wouldn't want the weight penalty over my 10D/10D Rev.
As for the zipper - Who cares about stepping on toes, the free market and competition is what drives innovation. Do some more customer research and if you think there's a market for it . . . Personally, I think a zipper defeats the function of a quilt, but then again I don't Winter camp much.