Lots of people are playing everything right now because SP is so easy to get. Most people who have been playing since September are now sitting with four rank 50 battle classes or more. When everyone can be everything the only way you won't have a party full of ARCs or whatever is by balancing the classes so that one DD class isn't more powerful than another.

I think THM is more powerful than ARC anyway. I doubt my ARC could solo the high rank NMs yet there's videos of THMs doing it on Youtube. I'm 50 ARC and 46 THM (my sig keeps going back to an old cache for some reason) yet I've no problem with nerfing them into line with other DD classes. It's easy to say they should boost the other classes instead but that would cause headaches for the dev team who has to buff all the mobs to compensate.