| >>Why not let the player dictate at what level they want to cast the spell|
There are a couple problems with this. The first is, it would require editing every spell, which is an enormous effort (300+ spells) that we've already done a LOT of times. I don't think anybody has the energy right now to do it again.
The second issue, which unfortunately makes it a non-starter, is that percentage bonuses cause a linear scaling. In many games that have stepped tiers of abilities, there is a flat bonus that becomes more or less irrelevant to encourage you to use the higher tier. Since we use percentages in most everything we do, a 5% bonus is as good at 100 ranks as it is at 1000 ranks - The difference is felt the same way. Increasing the buff from 5% to 10% would end up making the spell a lot better at higher levels in a way that makes things go really bad at high ranks.
That's why the higher-tier spells have more effects, not stronger effects.
However, you did just spark an idea.... I'll get back to you once I've floated it around a bit.