Won't be on FOP until the 19th...but regarding the 3 point "restraints". I imagine you have the back one for mainly support and something to lean onto if you need to. Maybe the ride goes up and you lean back? I'm avoiding ride videos so I haven't watched anything.
Then regarding the leg restraints..I imagine they have them due to the leaning. Do they only touch the back of your legs, or do they come around your leg more than what that thermal looking picture shows? If they come around more, I imagine it's to help hold people on the banshee. If it leans like that of a arcade motorcycle, then a lot of people's first reactions are to put their feet down from falling. Leg restraints would prevent feet from trying to go down to stabilize themselves and added support on legs would mean people aren't using upper body strength to stay on the seat.
So to me, it seems like the 3 point restraints make perfect sense for what this ride is...unless I'm completely wrong about the ride lol. However I'm sure the imagineers know way more than what I do, but I don't get why the leg restraints don't just stop once they feel your leg, similar to the stitch ride. Maybe it's a safety issue though...technology malfunction and now instead of stopping once it senses someone's legs, it keeps going and then you have an injury.
I'm sure the issue isn't as big as some are thinking though. I'm looking forward to trying it out when I go in shortly