Re: Magical Diversity.
I'd say that diversification for diversity's sake is good. There isn't always a larger goal in mind, but having the diversity ahead of time means you don't have to spend time developing it when you suddenly need it.
Whether it's worth it depends on how much you have to devote to getting it. If you're always designing your pretender around diversity or using Alchemy to empower in new paths, that's probably too far. Don't think in terms of focus, but of opportunity cost. What am I giving up to diversify?
And what will I gain? Which is harder to foresee. Getting a couple of low-level Air mages and a little air income may not seem like much, until Caelum invades and you can stop his raiders with cheap lightning immune thugs.
If a Skull Staff will get you into death summoning, you've got D1 mages, which means you should have a death income. Why shouldn't you get the Staff and a couple higher level Death mages? It'll get you into Tartarians in the end and who doesn't need those?