I think that someone asked a similar question awhile back on this also. My opinion is still the same (so forgive all those who read my previous).
I think that exercise is very important to all martial arts, especially the soft ones like Aikido. Here are some reasons 1) A healthy muscle is more flexible and can relax more as it is being used. If you haven't exercised in awhile flex your arm right now as hard as you can, and when you release it you will still feel tension. Proper weight lifting can help you learn body sensitivity to use only the proper amount of muscle to do a given task. 2) If your body isn't even in enough shape to walk a flight of stairs without gasping for air, then you won't be able to do randori either. I'm not saying that you have to become a marathon runner/Mr. Universe, but an all around program will help anyone.
Reading about O'Sensei, there are many stories about him lifting huge logs to get stronger and that when he grabbed onto your wrist for a technique it would leave bruises. So even this plays a part in Aikido, but we shouldn't "Muscle" a technique to make it work. I think it is a fine line between the two that all of us need to find.