The bottom line about the AIM's recommended procedures where they are not based in regulations (like this one) is the consistency and expectations that come from standard operating practices. That everyone, whether it's a bunch of pilots (with radios) communicating their position and intentions coming into a nontowered airport or the guy in a 172 flying into KATL, waiting to hear "contact Ground" by a Tower controller complying with his duties under 2-1-17and other applicable provisions of the ATC Handbook, are on the same page.
I won't disagree there are times to switch to Ground yourself. I've done it. Heck, I've also taxied to parking without calling Ground at all.
But if a facility wants to do something non-standard regularly for its own efficient operation, it sure helps to let people know. There are 2 or 3 towered airports, for example, at which the pilot is not supposed to switch to Tower when ready to go at the departure end of the runway. How can pilots be expected to know that if they don't fly in there all the time? Maybe it's the "Special procedures in effect.Notify Ground when run-up complete and ready for departure" on the ATIS?
But I guess that's way too much work to add that to the automated computer-voice ATIS for an airport that can't find enough time to say "Contact ground point six."

Actually, I suspect there is a good reason why they don't want to do that.
(BTW, out of curiosity, just spend some time listening to Live ATC for Atlanta. They don;t deem to be having too much trouble saying "Contact ground point niner" as part of the instructions for the traffic landing on the parallel runways 8L and 8R. There was one in which the Tower didn;t. In that one, the pilot, as part of the readback, said, Ground point niner. I like that "make sure we're on the same page" attitude.)
And, of course, the question wasn't being asked about airplane pilots flying into a hub. It was from an instructor asking what to teach his students.