I would argue that they probably shouldn't; in the sample characters in Hedge Magic runes get listed like Arts rather than Abilities. Likewise for other Accelerated Abilities like the powers of the Goetic Art in RoP: Infernal. Also, serf's Parma, but don't Methods and Powers on the sample characters in RoP: Divine and RoP: Infernal have specialties? They're basically what gets called Difficult Arts in later books, and they get listed like Abilities on the character sheet too.
In the saga where I play a Franciscan with Methods and Powers, we do allow specialties and I think that the description in RoP:D does specifically list appropriate specialties.
Personally, I think that it is OK since the advancement is so darn slow, having a narrow focus where you can add 2 to your "casting" total seems appropriate. Accelerated Abilites seem to have a narrower scope so maybe not in those cases.