The section regarding pilot acknowledgment/readback is in 2-4-3 and it DID just recently change, but not in a very significant way.
I've been in the agency for 6 years and the phrase "readback correct" has never been in that book during that time. I've always been taught, and always have, said those words in response to a pilot readback of an IFR clearance. But, I don't normally say it otherwise and I can't find any reason why we do it for IFR clearances either.
2-4-3 pertains to all ATC instructions. Do you always close every landing clearance with "readback correct," too? If so, why?
If everyone stopped saying it starting today, I bet pilots would think it was strange. But eventually we'd all get used to it.
I always get an acknowledgement from the pilot, with a clearance, and correct any incorrect readbacks as that's my job. If I climb someone to 14,000 and they say 12,000. I'll respond,"...negative climb and maintain 12,000." If they respond correctly, then I'll move onto my next task without saying "readback correct."
If I'm reading an IFR clearance to a pilot and they respond back correctly, I'll say,"... readback correct." If you don't respond to the pilot reading back a clearance, then how do they know if you heard them and/or if what they received was correct? Closing the loop in my opinion, whether right or wrong, I'll keep doing it when issuing IFR clearances. Why I do it in this case and not others, I don't know, but that's the way I was taught.
To play devil's advocate, if it is so important to have a hold short instruction readback, why is there or should there be a difference to issuing an IFR clearance? Shouldn't they be able to say,"DAL123 wilco" to you telling them to hold short of 27L just like they can when acknowledging any other ATC instruction?
Plus, if "readback correct except change route to read..." is in the 7110.65, then how come "readback correct" is not?