While my knowledge of competition in Tomiki and in general is quite limited, I believe the main reason we don't have competition in aikido (that was approved by O'sensei) is that it seems to go against the philosophy of it all. As an aikidoka you are learning to master yourself and to bring yourself and your surrounding into perfect harmony. This is why we learn how to be soft in our defences as well, so as to not harm our attacker. We neutralize their attack and restore harmony.
To have competition, you now are having people out there with an eye to overcome their opponent. Also, to my knowledge, you can't compete without someone throwing a punch or kick (or themselves) at you. To attack at all with an eye to overcome your opponent seems to go against the principles.
I am not saying Tomiki is bad... for I truly don't know, nor do I have enough experience to pass judgement on any ideas.
Just my thoughts,