I would contend it because if our dojo is a place where we train in a spiritual discipline (as O Sensei would point out) then it is of utmost importance that we come to understand that the dojo is a sacred room where subduing our ego is the main aim of our practice.
Others have addressed other points; I'll confine myself to the "if" of this "if/then". If
the dojo is a place where we train in a spiritual discipline, then
all manner of things might be said to bollow -- but is that true? Many people have advanced arguments that O Sensei intended aikido to be a spiritual practice. I won't take issue with that, but the fact is that most of those who came after him did not share that understanding. Whether or not they wanted to is immaterial; the fact is that you will find few aikido sensei today who could clearly articulate the nature of the spirituality that aikido is meant to cultivate, much less act as a competent instructor of the practices intended to cultivate it.
So, even if O Sensei intended aikido to be a spiritual discipline, it isn't a spiritual discipline today. Some people will talk in vague and self-referential terms about how spiritual their time on the mat is, but "spiritual" feelings do not a spiritual discipline make. For aikido to be a spiritual discipline would require that the curriculum include instruction into what the desired spirituality was, and the practices by which it was pursued. That doesn't exist; hence, no "spiritual discipline".