You are missing the point. Of course it is my choice. But the point of a reward based system is to reward certain behaviors. In this case: using the highlighted stats. "Just incentives" doesn't make any kind of sense. Incentives are how the game wants to be played.
So sure of course one can use the non-highlighted stats. Not really sure why you brought that up. Because then one might as well say they can play a farmer in D&D too - who needs to adventure - I mean there's nothing stopping you! Except that whole experience point thing. Yeah, that could be an issue... :-\
Sure Highlighting rewards what others want to see. Okay. What about what I want to see? What about character fidelity? Do we not care?
I'm playing a character that is an extreme pacifist. My character doesn't fight. The entire concept of this character is based around getting what he wants without resorting to violence. But the group decides they want to highlight Hard because they want to see this pacifist throw down. So now in order for me to get my reward I have to use Hard. So you are saying I can't play
Gandhi? Because I gotta say, using Hard would completely nullify the whole point of playing that type of character.
Would [fictional character] Gandhi be more "three dimensional" if another player had chosen to see what "your character is like when he's taking shit from no one" and Highlight Hard? Erm. NO! In fact I can easily argue the opposite. Why are your characters so willing to go against everything they stand for?
EDIT: I should probably say this has also happened to me. I was playing a pacifist Skinner.
If the group said, "I want to see Gandalf be sexy" would that be cool? (I can continue to give examples of characters that wouldn't do certain things and in fact would break that character's fidelity if you want!)
To use an off-system example it would be like other players writing my Burning Wheel character's Beliefs. Huh? Don't I get to choose how I want to have fun? How I want to be rewarded? Hell, how my character acts?
Or we could just say that the group has decided to screw me and say "you don't get xp this session". That's fun. Yay?
Looking at it another way, it is essentially like the players being able to tell me what my character's goals are! Yay?
Stat Highlighting is... how was it put? Ah, yes: "not a healthy dynamic".
EDIT: I didn't want to tear you post apart point-by-point, but I do want to call this out specifically:
If getting experience is more important to you than being true to your character, well, I think that says more about your own personal play style than it does about the game's intrinsic mechanics.
Again I couldn't disagree more. It says everything about the mechanics of the game. Because System Matters. And the system is saying: "Highlighting is how you get rewarded".