OSensei trained many different arts. The fundamentals of any art should teach you balance and stability. The one off situations or maybe they are more common than that....where someone falls into you or perhaps you put your foot down wrong and you go down. That is part of "combat" if you will. That is where you hope your training has taught you how to overcome the unexpected or unintended.
Oh sure, should, but I don't think aikido does really. Most of the time we're avoiding contact and/or not really getting our balance messed with too much while we're moving. That's why I don't think what I said can really be considered a one off situation.