I think the pace of the communication catches the new pilots off guard. If the busiest airports you have gone to are SBN or GRR. The first time you land in PHL or LGA is a bit of a *** experience. My first 121 flight was into PHL, I think the night before I studied the taxi chart for an hour, so I wouldn't screw it up. That said even after 4000+ hours in the right seat, I still jack it up no and then when I get something long and unexpected.

When I instructed at WMU I tried to make it a point to take a student into MDW (usually during instrument training). Hearing the pace of the radio chatter gives them an idea for further down the road.

As for an airline teaching radio ops, I think it depends on who you are hiring. I know large international airlines that have classes completely dedicated to it and if you are hiring 300 hour pilots, it may serve you well to take an hour and play some recordings of actual atc communications from around the system.

I also think sim sessions should be done with realistic radio clearances and speed. I can tell you at both 121 carriers I have worked at, 85% or more of the time we didn't even wear headsets during the lessons. So other than the PC, LOE, or MV, you weren't really talking on the radio.