Like Piratecat, I have a few powerful and crazy/evil spell-casters running around. One group of them in particular has allegedly been wiped off the face of the earth thousands of years ago, but are aware that there's a group that knows they exist and are secretly hunting them. Keeping their magical ears open for anyone mentioning them is a very rational security measure.
And when you're a creepy mirror lich, you have time to listen for anyone in the modern day pronouncing your name.
IMC, most custom spells aren't available to anyone, but for my own satisfaction, I like to have the system worked out, both so the spell works in a consistent way (which is why I worked out the fey-to-gold spell on these boards months ago, for instance) and so that, one day, the victorious player characters can find the spell in the Big Spellbook of Evil Badness, even if they're a long way from casting it themselves (and may never be able to over the course of the campaign).
