Not something I’d want to see occur in canon, but it gives MOCists a ton of freedom to enjoy: You like the aesthetic of a certain kanohi, but you really think your guy would have a different power? You have a ton of creative license to work with, so you can have at it.
I think overall the pros outweigh the cons; as far as why it doesn’t show up in canon (with the exception of the Hagah), we can just say it’s one of the social mores of the MU that barely anyone’s ever broken, kind of like how in the movie The Patriot, we see the British forces (underhanded as they are) treating warfare in a similar manner to a live-action game of chess, assuming that their opponents will follow more-or-less the same rules of combat that they do, which makes them particularly vulnerable to the guerrilla-warfare style of battle that the protagonist uses and teaches to his fellow Americans.
Also, as for the Av-Matoran kanohi being considered the noble versions, I personally like to think of them as being what the noble versions look like when given adjustments to be better-suited to flying, having the fins and vents added-on to be more aerodynamic (and maybe also to help the wearers breathe whilst flying at real high altitudes and speeds) similar to how Nuju’s kanohi matatu having a scope is the exception, not the rule, and the scope was affixed to it as a modification after it was crafted; as for what I believe the regular noble versions of the Mata’s kanohi to look like, @MaskMaker/KhingK has done an amazing job on the akaku, and I imagine the regular versions of the noble hau, kaukau, pakari, miru, and kakama to have the same aesthetic that he has successfully conveyed:
