Why would you even try to shoot 9mm in a revolver built like a tank?
You've answered your own question. Shooting 9mm in a Blackhawk would much like shooting pleasant .38 special rounds - except that the 9mm ammunition can be bought dirt cheap, in bulk off of the surplus market.
- A nice, inexpensive plinker.
I doubt that anybody will be doing any competitive shooting with the 9mm cylinder, or using the pipsqueak round for hunting anything bigger than a rabbit - but as an inexpensive plinker for informal shooting it ought to do just fine.
Those who disparage the 9mm cylinder for being less accurate and less powerful seem to forget that the gun comes with a .357 mag/.38 spl cylinder too. - So if you are all done with plinking and want more power or accuracy, well about five minutes later, after a cylinder change, that is exactly what you will have.
So, I'm not certain where all of the negativity is coming from but from what I can tell, it appears to be somewhat misplaced.
The convertible Blackhawk is what it is - No more and no less.
Would I pay a bit extra to have the ability to shoot cheap 9mm ammo for plinking fun? - Sure, why not?
I am sure that there are some preppers who would buy this model with ammunition shortages after the apocalypse in mind, and to some extent they might have a point about that. A dubious point if you ask me, but a point all the same.