It may not appeal to everyone, but IMO a redundant release system seems like a good thing to me. Sure QR don't fail that often but no one can say they are %100 reliable.
It is certainly a judgment call based on personal preference. There is no real wrong answer to debate. We just have different perspectives worth considering.
Having 1, 2, 3 or 4 QRs in your set up does not make it "100% reliable". Nothing in kiting is 100% reliable. I am assuming you know that too and used "100%" to illustrate your point. There is no doubt that 2 QRs are better than 1 if all else is held equal. However, added gear like the Mystic spreader bar adds another pieces of gear that may snag a line, fail, add maintenance time, increase costs. there is a point with any gear where redundancy becomes a hindrance. In my judgment, more than one is too much,
I see the value in simplifying, reducing costs the cost of your set up. Safety is increased because you will have familiarity with your QR that does not change when you buy a different bar. One more factored I want to add is having the QR on the spreader bar also gives us more choice. Currently, if we like a depower setup we don’t really have a choice to also pick our preferred QR. We must accept the QR that is sold with the depower setup. If kite gear manufactures separate the QR from the bar we can buy the depower system we like the best and the QR setup we like the best.