This is an aspect I've always enjoyed reading about as well while studying Polyamory... Not even about the sex aside, it allows for relationships to exist that do not have to "change" the individual. The thought that "I'm having a relationship and I can still be totally "me" without censorship or "changing" my personality" is one side of a poly relationship or a marriage that I find interesting to my overly-logical mind.
I've seen friends change from what they were to some sort of hyper controlled "mix" of the couple, ultimately losing some aspects of what made them fun to be around or even blocking their self-expression. Some would call that (for guy's changes after) being "pussy whipped". There's identity loss, and just existing to support her. Which I guess is ok for some.. But then give a year or 2 and the woman wonders where the excitement went from her partner...
Have seen that happen both ways.
The aspect then of Poly thought in allowing each individual to be themselves for who they are without forcing changes (and allowing more logical thought in handling issues and events) really has intrigued me because of this.