Some of us have issues simply with sitting still, including chronic pain. For us there is walking meditation. Or aikido. That's all I'm saying: I can't see how zazen specifically in and of itself is necessary for aikido or vice versa.
I don't think it is. I think that it's one of several (I wouldn't say many) esoteric practices that, when pursued in a disciplined fashion, can help one's aikido in an indirect way.
You might also make the argument that without some kind of practice that gives you training in being fully present, you'll be unable to do aikido (or a lot of other things) beyond a certain level. The better your training in these practices, the better you are able to function in a situation where a lot of stuff is going on at once, and if we take this a step further, as you train or try to use aikido in increasingly more challenging situations, eventually that ability will become necessary.