Calling this a benchpress is like calling kipping pull-ups real pull-ups.
I definitely understand where you're coming from GS.
Its about using advantages:mechanical and technical, to get more work done.
Sooooo, if you arch your back, (your booty has to stay on the bench), you make a shorter distance to travel and increase your mechanical advantage. It allows more than JUST your chest to work for you. it adds in more shoulder, and triceps, even biceps. I know it sounds funny, but I've woken up the day after a bench press workout and my biceps were sore!
The same thing can go for pull ups, or kip ups. Think about this, by swinging, they get more leverage and carry their momentum. Sooooo, they can get more work done. Instead of being able to do maybe 4 pull ups, they can do maybe 10 with a technical approach?
The bench is the same way. A person might be able to do 200 flat back, no leg drive, and no setup. BUT, if they add in the setup, (their mechanical advantage), they might be able to get 225.
So I think the two are similar. Its all about approaching the lift, or movement with technique and a plan of attack.