[Illuminated One]

I didn't say you should try to get those spells early (if you want to know it I'd go down thau and alt first), it was merely an example how Yomon struggles to get them. I have an error in there and the chance is even lower.
You have a ~ 2% chance that when you want to use lightning bolt that you get a guy who can cast lightning bolt. Same for most other spells. So quite simply you need 50 Yomon mages on average if you want to spam lightning bolt from one of them (yeah, communions, but you can't have a large communions running around everywhere and fatigue can be quite ugly if you are casting spells that your slaves don't have the paths for, have fun calculating the fatigue with everyone different). Basically it's not you deciding which spells you use but the drn.
It's not a question of theoretical availability it's a question of mass. Sure you can go through your mages and look at what you have picked up, but this is a) tedious and b) subpar in most situations (farstrike is defeated by missing, shield, armor, same for magma bolts, bow + quickness only works half of the time and don't think of using crossbows, etc).
Now consider Bogarus who can have lightning bolt from every mage they recruit if they choose so, or nether bolt or magma bolts or via smallish communions a lot of good spells like magma erruption, gifts from heaven, shadow blast, astral fires, dust to dust, leech, etc. Same versatility but a better control.