In other words, delete all caster classes and penalize hybrids? You do realize there are enemies strong in Element X, and every single mage has to at least dual-class, right? Not a fan.
For dual-class you are penalized by one point. You have two schools. Where is a problem?
Did you get used to Hex-o-class e.g. Jack-of-all-trades as a staple DOS build?
Did you noticed that any mage could use a tons of wands and a staffs with any element and any spells?
Did you notice a hell lot of scrolls and grenades?
Did you noticed that on the level 7 your inv is flooded by consumables without use?
Looks like, such a rule would bring at least some meaningful decisions in builds and resource usage, isnt it ?