Archimedes,
The problem is that the 'system' isn't uniformly applied.
If 617 and 120 were special cases, then so too should 19 and 92 have been (especially 92, as the most successful RAF fighter unit in WWII), and I really shouldn't have to explain why 43, 56, 74, and 111 deserve to be saved.
I'm aware of the histories of 3, 11, 6 and 1, but they're not in the same league.