I think that you may be misunderstanding or misapplying the rule that you mentioned, and you are also disregarding other rules. A positive charge on a nitrogen atom that has a full octet is frequently observed in various Lewis structures. All else held equal, I would rather put a positive charge onto an atom having a full octet than a positive charge onto an atom that did not. There is also the question of aromaticity that wildfyr mentioned. Perhaps you can start with these two ideas and explain why some of the resonance structures are minor contributors.