Paul, aerobically train in aikido, you're a genius! We can start new classes -"aikidosise" and bring the dojo numbers up (and I can get that nice little lycra mini-hakoma at long last).
Funning aside, yes, I agree, for competence in any sport/MA etc, only practicing that sport or MA can achieve this. However, sports training has come a long way in the past 10 years, never mind 25 (compare athletes today versus then). While you need to practice aikido to learn how to use your body correctly in aikido, other training can help you more efficiently target specific muscle groups you use. This is a regime used by most professionals, so why should MA's be any different?
As for aerobic aikido classes, I would hesitate to even imagine their format (1000 sword cuts on the bounce, work that jo?) and could see trying to run such a class an easy turn off for most aikidokas whereas the stamina gained from aerobic exercise can be performed nice and toasty in the gym.