Hi guys,
You have pretty much knocked the nail on the head - Judge Dredd is intended to be a self-contained game, rather than one where you can immediatly port what you like all over the shop. That said, porting is not that difficult to achieve - take a look at the dinosaurs in the Creeps chapter, as they were included to demonstrate how to bring creatures from the Monster Manual into your games. No changes required, aside from DV, and that is a simple matter of looking up the Reflex save. As for game balance when porting stuff over, just run a few figures in your head - it is what Games Masters used to do
Judges were always meant to be tough, as we wanted to give players the type of character who could have a lot of grief thrown at them and still fight through. That said, once Citizen characters hit mid-level and beyond, the balance starts swinging the other way, as judges are, ultimately, limited. No new equipment for them, and only ever one prestige class. Citizens can buy all the gear they can afford and hop around classes like they are going out of fashion. Not to mention starting their own street gang.
I have seen one or two criticisms levelled at Dredd, and it is always about the Judge class and its portability. That ain't the point of it! That said, I thought we would always get more flak about using DV and Reflex instead of AC - I have heard absolutely nothing!
Each to their own, I guess - but we are always interested in pushing the d20 boat just that little further out and trying something new. . .
Hope all that waffle helps!