Probably someone is going to die with one of these deployed, as it slips out from under the arm, and becomes tangled with the lines, creating a deathloop situation that can't be ejected from. Yet another thing to keep track of as you try to wind up a downed kite. Board, lines, both hands busy winding lines or pulling in the kite, managing your toy at the same time, which you deployed instead of thinking your way out of the problem because it was there on your belt and you spent a hundred bucks on it and you were itching to use it. Bottom line, this thing needs your hands or arms to use it. You can't wind lines or do anything with it, you can only cling to it and use it as a crutch. If you can't float or swim some distance unaided, you either have no business in an extreme water sport, or you need a PFD.
Barring a viscious current even a mediocre swimmer can swim miles with the proper technique and time. Take a breath and fut your face in the water, let your arms and legs hang, and don't do anything. Relaxing, you don't sink, even with no wetsuit and a harness. You aren't burning any energy. Do a couple breast strokes, take a breath, and relax. Rinse and repeat. Or float on your back, and do the same thing.
Kite is down, can't relaunch, kite pulling you where you don't want to go? Throw the QR and recover the kite, and use it as your flotation and self rescue to land. Leading edge broke? Recover the kite, the bladders are still inflated. Roll it into a raft and Gilligan your way to shore. Had to release the kite and you are in the middle of the fookin ocean without a proper PFD or beacon? You are not the brightest tool in the shed, deploy your tool and hope someone in a plane can see you. Kiting in double overhead waves half a mile offshore without a PFD? You deserve what you get.
Either way a PFD is completely better in every way, and also allows you to keep your hands free while you swim, manage your situation, find the kite, wave for help, whatever.