I am using my clip-on RED mattebox with all my Canon glass, (including telescoping zooms) and I simply bought a 110mm-XXmm mattebox adaptor for each lens and leave it installed.
http://www.bhphotovideo.com/c/produc...Down_Ring.html
10 seconds to remove the box from a lens and replace it,
with filters, on the next lens choice. Your proposed system would require that you either unscrew filters from the lens every time you change it out or buy multiple filters. I also think that you might be oversimplifying the use of a flag by assuming you have only one light source. A mattebox assures you have the lens shaded from almost every angle, simultaneously. If you simply set up a shot with no movement and you have only one source of light, sure a flag might work fine. Try to follow on-screen action or change angles and you would need a grip following you around with that flag to keep shade on the lens.
BTW, when I just don't want to deal with packing and carrying a MB, I use the Flex Lens Shade and screw on filters. Works pretty good, but definitely slows things down.
Hope this helps you see why a mattebox is much more than just looking cool.