Okay, explain it to me. How does an option you will never use or be encouraged to use or have a reason to use affect your experience?
Zanos didn’t provide an answer to that so that is still a mystery.
Okay, I'll try. The key reason is that by making something an option, you send a message. This is especially true if you deliver that option via an npc in a story-rich game. "Have you devoted your life to Shar and having second thoughts? Has the sight of Halsin's bulging muscles made you green with envy and wishing you'd spent all those years honing your physique for battle instead of praying? Worry not, because we at <Spoiler_name> inc. have you covered! For the low low price of 100 gold you can change your life in an instant and become the adventurer of your dreams! What's that? Fighters train for ages to be as good as they are? Oh please, that's downer talk! We don't do that here. We have already notified your loyal friends and mortal enemies of your unexpected transformation and in the spirit of politeness requested that they refrain from commenting on you suddenly swinging swords instead of spells. Oh and don't worry, we won't tell a word to Shar - you'll still be her favourite errand girl!"
I am exaggerating for comedic effect so please don't look too much into it, but see my point please - allowing anyone to simply change their mind about something as integral as their class communicates that the above example is something normal that can happen in the world and isn't even worth noting by its inhabitants. Making classes changeable mid-run removes nearly all commitment and renders that choice casual for both the player and the writer. I no longer have to carefully consider what class I want my character to be and all the unique class dialogue and reactivity may as well refer to my clothes. Is this something you can relate to at all, or are we of completely different minds?