I disagree with all of this, This is something that is super easily enforced, We make people take down hobo shacks larger than a 6x6 every single day. We have a whole addendum to the rules for special areas you can build outside of the door. It is really easy to stack yourself twice for the two shoulder widths. This is a standard rule on some other servers. Lets be honest as well what type of loophole/misuse are we talking about? I really can't think of a single way that this could be misused.
I also disagree we have unused possibilities. Look I build bases that are pretty creative I would like to think and I am running out of ideas that aren't just more of the same in a new spot. I have built in every single area that is unique or cool. This would open up tons of creative ideas.
I do think that instead of doing shoulder widths it could be managed by block lengths i.e. can not be longer than a 1x3 extended out.
Sorry if this came off as aggressive, I just think some players on this server are too stuck in the norms, Its a vanilla server where we aren't going to get massive content updates ever and doing small things like this motivates players to come back, and engage in the server more.