In Sweden we also have this PDC system at some airports. Don't know if it's the same as in the US, but in the swedish system, pilots never speak to clearence delivery. They get the clearence via ACARS and then just press the button to accept it. Next call they make is to ground.
About readbacks... very surprised about this "roger"-thing. The situation in Sweden can be good described by examples like these:
GND: Spainair 123, go behind the Scandinavian 737 at your ten o'clock.
JKK123: Roger, JKK123.
GND: JKK123, readback please.
....
CD: SAS123, Cleared ***** via AB, ABCDE, UN456, FL80 bla bla bla.
SAS123: Cleared ***** via AB, ABCDE, FL80 bla bla bla. (the same but not the airway)
CD: SAS123, negative, after ABCDE, cleared UN456.
SAS123: After ABCDE, cleared UN456.
CD: SAS123, readback correct.
....
So I'm having kind of hard to understand how you can readback a long and important clearence with just roger and still think it's safe (if it's not given as PDC or similar way)...
And last about the after/behind conditional clearences... I've learnt it's supposed to be...
"Behind DC9 on short final, line up runway 26 behind."
