This will probably sound crazy to most of you...but the only thing that would hold me back about couplers is this: I am always paranoid about training my body to respond wrong under stress.
If, for instance, a bunch of paper targets charged me at the range, say, and I had to lay down a lot of fire and change mags under high stress, I would be paranoid that I would end up chucking out an empty mag with a full one stuck to the side, wasting 10 rounds and one slot on my chest rig, or else lose valuable time trying to move a mag over that wasn't doubled, or reloading an empty, or something of that nature.
Now I know that probably sounds crazy but I wouldn't mind to hear from some guys who are used to shooting at multi-directional ranges about whether they would use couplers, and why or why not.