I wouldn't say that it was common - let's not scare people - but it did happen a few times.
The idea of strengthening the area by substituting a big C-channel for the smaller L-channels always seemed good to me, but to my knowledge, no one ever did it. Instead, the universal solution seemed to be to close the crack with a clamp, tack-weld it shut, and then weld a sturdy steel plate across the break area. I seem to recall that each break was covered by plates in a couple places when both sides were accessible. Never a problem again.
I would not presume to tell a welder how to do this. They are the pros, and I know very little about welding. Yes, it is kind of unnerving when you first discover it, but unless you know a lot more about welding than a professional welder does, I suggest you put it in his hands, let him see what is happening, decide how to fix it, and turn him loose.
My 2002 2720SL was one of the ones that developed these cracks. I checked the phone book for Scottsdale, chose a welding shop that looked good in the ad, and went down to talk to him without the TM, but with a couple careful photos in hand. IMPORTANT: At this point, be sure to explain to him that if the TM is opened, it cannot be moved. There is probably no reason to open it, but it is best to let him know in advance. The shop I chose had a back-in parking space ready for me, just in case it was needed - it wasn't.
The guy I initially chose gave me confidence, so I took the TM to him. To avoid unintended consequences, I showed him how the swing hitch worked (he had never seen one), and how the WDH worked (he had seen hundreds of them), and turned him loose. Next day I drove the repaired TM away, with never a problem again.
Again, let me emphasize that this is not a complicated situation for an experienced welder, especially one who works on vehicles. He does this every day. No elaborate procedures are needed. I would certainly not try to instruct him on MIG-MAG-TIG, or choose steel plates sizes and placement for him.
BTW, I know I am dating myself by suggesting that you "consult the phone book". That was long ago. I know you can handle whatever today's equivalent is.
Bill
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