1) The spell applies to specific creatures – this is a special case, and there aren't many creatures with magical protection.
2) I don't see anything wrong with having a couple of offensive spells in the summoning list. The school itself is quite different from Chaos.
3) Chains are only relevant for warlocks, which is also a special case.
2. That's the point, nothing bad, it's just magic from a different school, so that some mage can also deal direct damage, at least a little.
3. They are relevant.
Lightning and Stone Skin have different damage and different attack types + different bonuses + different artifacts affect them. Thorns are more of an area-of-effect spell...
Each spell is for some "special case." Isn't that the point of the difference? Why debuff the enemy's attack when you can hit them with lightning and reduce the number of units in the stack, thereby reducing the damage? Why buff your defense when you can just as easily cast Meteor Shower?
Except the ones we've made