I love climbing with two separate systems when needed. Positioning possibilities are tremendous! But it's not something I do as a matter of course as one rope, set properly, works just fine. I tried two bridges for several months and did not feel that the extra bridge added any significant benefits. I trust the strength and robustness of my single bridge as much as my single rope system. If I didn't, adding two wouldn't help.
I don't disagree with any of your points. I don't use two ropes because I the rope isn't strong enough. The rope itself is probably the strongest, safest part of the system. I use the second bridge because I use two systems and it helps with the angles. I use two ropes because of the "what ifs" I believe should be considered when trying to mitigate any risk. I think the weakest, or most difficult to qualify piece of the systems we climb on is the tree. I like to be anchored to two different parts of the tree. Here are some real world situations where having two systems may have helped:
1. I read an article about a climber in a competition who was using a ring made by a company who also made shower rings. A shower ring was sold as a climbing ring. The ring broke and the climber fell. Defective hardware - second system may have helped.
2. A youtube climber was ascending when his tie-in point broke and he fell. Second system may have helped.
3. August Hunicke says he fell when he unknowingly cut a length off his MRS tail. He was descending, rope ran right through his Blake's hitch and he fell. Second system may have helped.
4. There was an article last year about limb walking with the rope runner pro. A climber fell limb walking when the angle became horizontal enough, and the tail of the rope was not "down" enough. The climber slipped from the branch, and the device didn't grab. Second system may have helped.
5. Another youtube climber was using a lot of rope in a re-direct, and then descended. Rope ran through his hitch hiker and he grabbed it before he fell. Second system may have helped.
6. Another youtube climber had just cut out the top, and only the spar was left after a long day. He secured some type of canopy anchor for SRT to descend, stood up in his picks to release his lanyard, didn't see the anchor pop off the top of the spar, and down he went.
7. Ever have your system freeze, lock or stick with Sap? Descend on your second system.
8. Second line is in the tree if someone needs to rescue you.
Defective hardware, operator error, and tree defects. These risks may be mitigated by a second system.
I don't use it every time. I understand you climbers that have been using a single line for 20 years and know how to safely navigate the risks.
I'm not lobbying for everybody to climb the same. But these are my thoughts behind using it. Thanks