Don't forget about the soul stones themselves. They are crystals, and if anything in FF has taught us, is that crystals hold immense power. There are also multiple crystals for each job with the memories of the past holders.

I mentioned this in an old thread, but we're adventurers, not white mages, not bards, etc. We use the jobs as a means to an end, but they are exactly as their title explains: jobs. Vocations. Since our vocation is Adventurer, we don't have the time to invest into learning every ritual, every song, every technique. We just need something that works and that will keep us alive.

So we cheat. The soul stones give us the memories and powers of the <job>s of the past, not the Echo. If it did, we would have access to those powers all the time instead of just when we hold the soul stone. It's for this reason that there is always someone better at our job than us: they actually devoted their lives to their craft while we just...cheat.

EDIT: Sorry for the necrobump! hehe...