As you know, Power Integrations also serves the market of "companies who want a power supply but can't afford to permanently employ a sophisticated PSU designer forever"
...what I like about Crutschow's circuit is that there is no doubt whatsoever what is the current gain, and its easy for anyone to work out. And it has the advantage (also as does the single BJT solution) of drastically reducing standby losses especially when the vout of the flyback is high.
Though Crutschow's cct has well tolerance gain, unlike the single BJT method, which even if there is ways of reducing it, its still not defineable, at least not yet....and Voltage mode control dynamics are such that wide tolerances of gains in the feedback loop are not wanted....especially when many of the Power Integ. users are not dedicated PSU people.
The problem is deriving the small signal transfer function of Crutschow's circuit. Once that is done, I believe Crutschow's circuit will go into Power Integrations Folklore.
As far as I can see , Crutschow's cct has the same features as others with a grounded opto-emitter, ie there is the gain term of the 931 ohm divided by the resistor in series with the opto...and then its just the gain of seven.......so I think that's it, then its all the same as the original one on AN-57. But I would have to check.
If I were Crutschow i'd get a patent on that circuit, as it is a cracker to make up as a module and flog with the topswitch as a method to reduce standby losses with high vout flybacks using topswitch, which is the best selling offline monolithic flyback controller in the world