I agree that a dog who is incapable of being in the general environment of a trial without acting in an aggressive manner should not be there. It isn't fair to that dog, and it isn't fair to other competitors.
HOWEVER, the missing part of this is that many, many dogs in agility - and in life, but particularly agility because of the breeds who are popular as agility dogs - do not appreciate dogs who are hard staring, invading their space, or behaving rudely. They are otherwise completely capable of being there, being comfortable, and being no problem for anyone. Asking people to keep a brain in their head, not crowd at entries and exits, and ideally maintain reasonable distance (8 feet I think someone said) between their dogs and others is important.
I absolutely believe a dog should need to be able to not stress out other dogs or behave badly at agility -or public, period. That means not lunging, growling, or barking like a fool at other dogs (and possibly traumatizing them or stressing them out)- but it ALSO means not creating issues and behaving badly with exuberantly friendly behavior, or being allowed to wander into another dog's space/leash range (and possibly traumatizing them or stressing them out) because the handler checked out or just doesn't think anything's wrong with that.
I would much rather deal with a well managed reactive dog who's owner is paying attention to their dog's needs and space at a trial, than the owner of a happy go lucky exuberant dog who believes their dog cannot possibly be a problem and allows it to greet every other dog and person in the area.