It's somewhat hard to explain
On one side it's because of "book-keeping"
This game is convoluted enough as it is without starting to break down perks into profession-dependent variations. I know that this is a very tiny point to make and one that perhaps will prove not to be an issue in the future at all. However, being an "idealist" or "purist" or whatever I could label myself, I would rather go for a more straight forward and basic solution if such a solution exists, and I do think there are better solutions in this specific case.
The other side of it is that I want this to be a skill-based game, not a game based on larger things like classes. To give an example, WoW is based mainly on classes and secondary on stats. What you do and how you perform in various scenarios in WoW is based largely on what kind of class you are, not on how you set up your skills, as WoW doesn't really have a skillsystem like in AO. I want perks in AO to be based purely on skill-system. If we start dividing perks into a profession-dependent system then professions in AO would have taken a step towards becoming WoW-classes.
You might say that it doesn't matter if crats have 100% def checks and higher AR or 80% def checks and same AR as they have now. Well, when it comes to crats chance at landing perks in PvP, no it doesn't matter. The key difference for me though is that the first option is based on crats needing special assistance because they have gimp AR, while the second option is based on crats not needing any special help with their perks because they have good enough AR to deal with 100% def checks. Basically, the first option is based on what type of profession they are while the second option is based on how the individual profession performs. It boils down to me wanting to avoid officially classifying a group of professions as "support professions" by giving them a handicap-entry into perks. That handicap-label is only going to come back to haunt these professions again and again. Instead of labeling them as handicapped and giving them a motorized wheelchair, I say remove the handicap if you can, and it's easy to do so in this case.
I just want stuff in AO to be based around individual professions stats or individual characters stats. I don't want anything to be based on groups of professions being put in a box and labeled as something or another.
I guess I'm scarred a bit from traders being denied good damage back in the day because of labels. Be it support profession labels or "shotgun don't work like that"-labels.