Don't worry about 'deleting' useless spells. Sure, you might later get more powerful or 'supped-up' versions, but you never know when you might need some of the less powerful ones.
For example, suppose you're in battle and your magic is getting low, too low to make use of, say a 'greater fireball'...BUT you still have that teeny basic fireball spell and there's sufficent magic to use it.

And in battle, especially say after casting spells to make an opponent vulnerable to fire, frost, ect. often it takes just the right blast to knock 'em down.
You're correct, for the most part you don't need 'em...but in a crunch those spells use less magic than your more favored mega-spells and could prove to be a back-up. You never know...