I could've swore that Devs stated that they're planning on giving Retainers the ability to use Job-related gear so I'm not sure how they're going to do that. If you main a class to cap and your Retainer is the same thing it would be counter-productive if you had to give up your Soulstone to allow them to use gear. Maybe they get something else in retainer or there's an option to have them BECOME the job via the Retainer Option menu. I don't know.