I've got the Armageddon Gear bag that attaches to the Mini Gun Plate, but rarely find myself using it unless I'm running a tripod in the rear. I really prefer placing the bag exactly where I want it, with a bit more "squishiness" from a Gamechanger that settles a bit better.
Although I probably use the full-size waxed canvas gamechanger on 90% of stages; I carry around a few other specialty bags for the 1 stage that might present a more interesting problem. In those cases, I have the Wiebad Milk Toast for small ports, the Tab Gear small rear bag to attach to my hand for heavy movement stages w/ fast time limits, or the Wiebad pump pillow for stability.
Having said all that, I'm mostly just using 1 bag per stage; these others just provide an option if the stage design calls for something specialized.