I don't think that you are wrong. I've seen good trumpeters who leak minor (understand negligible) quantities of air and still sound good. However, I guess that even such a leak would make your playing less efficient - means that you probably put more efforts than necessary. As the quantity of leakage grows, the necessary efforts to keep the sound and pitch up grow up to the point where it may get impossible to compensate. I would agree with the above, that if the leakage is significant, the soft long tones and slurs may help.