Just because some Native Americans didn't use a floppy rest doesn't mean that it isn't useful or superior in some ways for what you are using it for. It could just be that by the time us Euro-Americans started paying attention, rate of fire on horseback was more important than pinpoint accuracy from the ground. This, after all, is the main reason to hold multiple arrows in the hand -- combat, not hunting (unless you're charging bison from horseback). It's hard to tease these things out -- especially being so far away from the bow's practical use.
Here's my thought, which you can take or leave.
If firearms are the more recent progression of projectile weapons (starting with rocks, javelins, atlatls, etc.), then it makes sense that bows would be used in as diverse ways as firearms are used today. A Glock is different than a 300 Win Mag but both have their purpose. Just because you don't usually find a scope on a Glock doesn't mean that it isn't a useful addition if you want a single, well-placed shot from your G20. Shooting arrows off your knuckle is cool but its usefulness really shines in rapid-fire archery or even wing shooting. Less so with precision archery. A floppy rest is a small change that can make a big difference in consistency -- at least for me. But maybe that's just me. I'm no expert with the bow so maybe I just need that little bit of help.
You may want to take a look at some different styles of archery from the east. Granted, they're often shooting off the other side of the bow so they can rest the arrow on their thumbs or index finger (bow hand). But there's still a lot to learn there. Much of that is still combat archery. But there's a good, rich tradition of target archery among traditional archers in Korea, Mongolia, China, Turkey, Hungary, etc. Even in combat, a bow was used in many different ways (horseback, marksman, volley, etc.). So much to learn, so little time.