Thinking about it, I'd still get the appropriate gear and spells learned anyway despite the roleplaying I'm doing. A few extra gold for something I'd probably never use alone, but once a group comes up, you can't just ignore it.

Like you said, a Warrior asking to raid, but refusing to use shields, is a bit of a painful idea. Still, a neat RP element would be having him use a shield, but reluctantly in doing so. ["I may not like to use these things, but I suppose the time has come to put that aside. If it'd help you, comrades."] Same with this Holy Priest and Mind Control business. ["I may not believe in the Shadow, but if it'll save my group, then I must tap into its power."]

I mean, there are some spells I bought on my Hunter that I never use. Doesn't mean there's a potential for me to get in a raid that WILL use them.

It's all about being prepared. Same with an RP character.
The challenge of an RP character would be to create a convincing story as to why you must use it now after swearing it off for however long you'd been alive. Heck, make that point of your story a plot point, like tapping into the Shadow spellpower of the Priest would turn them into a Shadow Priest for a time or using a strong amount of Fire magic, like Pyroblast, would harm your Frost Mage, a la High King. *shrug*