Stretch is great, but too much stretch can be hard on the back - especially those of us with giant, freakishly large children. If the baby stretches and or leans away from you, it pulls on your frame and can hurt. Any stretch needs to come with support or with a limit. This is why you don't see 100% cotton stretch tube non-adjustable pouches. The stretch is almost limitless and a heavy child just pulls the sling out of shape. 95-99% cotton 1-5% spandex (lycra) woven material makes what I consider almost the perfect amount of stretch for a pouch. The stretch conforms around mom and baby so nicely and there's limit, so the pouch won't stretch out of shape.
Also, if your baby is a little pipsqueak (or clings to you like a monkey), you can probably get away with more stretch.