I think that's a good way to do it. Each spell should have a required skill level to be 100% chance to cast. I don't think skill levels 25/50/75/100 is the way to go though, the skill requirement could instead be any level from 0 to 100 depending on the spell.
And by removing the 25-skill-point milestones for "cast without fail," then one paves the way for a really interesting "perk" system more in line with what melee combat got: various special effects at those milestones instead of "you can cast the next level of spells, yay, big whoop- the Fighter next door can paralyze people, the thief across the street can jump off the surface of water, and you got a stinkin' spell upgrade!"
Just example ideas:50 Destruction skill (Journeyman) perk, Fire damage spells have a chance to set the target ablaze for damage over time. (Possibly set amount based on Destruction skill, possibly percentage of the spell's damage...think about it)
75 Destruction skill (Expert) perk, Frost spells have a chance of freezing targets. (Even if only for a second- think 1-second paralysis spell effect to answer
"what the hell use is a second?")
100 Destruction skill (Master) perk, Lightning spells have a chance of arcing to other targets. (Potential double bonus: suppose the devs in their genius have added some sort of arcing lightning spell that jumps from target to target,
and a Master of Destruction still gets the perk for some potentially graphics-card-taxing lightshows with truly awesome destructive power.)
Or, we can stick with coin-op-game never-fail spellcasting. :sick: