I'm the opposite. I use my MDS all the time and don't feel it's useless. I get better empty highway mileage than our lead guitar players 6.2 Ferd, and way better than my bro-in-laws 2012 chevy 6.0 ever did. He traded that in on a Dirtymax because the mileage was about 12. Get about the same as my father in laws 2012 ecoboost max tow. He isn't very impressed with the Eco, but likes the boost.
The flapper's function in the muffleris more than likely similar to the performance mufflers with a flapper. Noise reduction. At idle and part throttle the flap is closed, causing the exhaust to take a longer path to exit the muffler, reducing volume when maximum performance is not needed. Waith more throttle the flapper is forced open by exhaust pressure giving the exhaust a less restrictive exit, freeing up some power at the expense of more noise.
The resonator is for mds drone.