First, you are assuming the bolded part will always happen. I am not.
Furthermore, if it
does happen then I would say it's a problem with the player, and if that player is ordered to roleplay a certain way, they will act subversively anyway. Starting with the premise that not everybody at the table buys into the table rules does not make for a sound argument.
But to answer your question more generally, i.e. what is
good about letting players decide things about their own characters, how about:
- Everybody gets to play the way they like, increasing overall enjoyment for all.
- Nobody has to police how other people are playing.