Coming from a background of Judo and Karate (as well as that well known Canadian combative art Jr'A'hockey-do) I am of the opinion that there may very well be a real advantage to prior martial art training when first coming to aikido for most people.
I was however fortunate in that most of my teachers stressed basics (Kyhon-Dosa, Kihon Giho as well as atemi application) so that I am open to the arguement that a good teacher/dojo that teaches the correct delivery of atemi and spends time on the physical realities of the techniques (eg. blocking, stance, distance, moving off line etc.)could negate the advantage of having prior martial art training for some or maybe even most beginers.
In fact first hand (first black eye, first busted lip....)experience with military combative training has demostrated the martial abilities of certain, all be it very motivated, people who had no prior martial arts training.