Well as I see the lore on soul stones, and seeing they are in the lore of many quests, Soulstone are a mean for the warrior of light to access abilities and memories of said job. For a normal person they would have to train in said martial progress. IN each of the stories and quests none but the warrior of light are able to truly unlock its secrets quicker then anyone else. For example in the paladin quest, when the stone only shines with you around. That does not mean you cannot be a job, because you can you have to find your way to learn such abilities. Plus like someone else said earlier on this post it is the way the warrior of light can access the other said abilities as a mechanic. In addition it can also be the power of the echo that allows you to access the abilities, a good example being that the warrior of dark have jobs assign to them. So lore wise it is the warrior of light that has the right to use the soul stones, but that does not mean a player cant be a job thru training. A good example of this is the elementals and thamaturges guarding there secrets. Nothing in lore says you cant be a job. If that was the case then there would have never been any of those in the first place.
Light and Dark? We all have it! 
