Hmmm a mystery. I don't think offensive spells are the only reason here. I'm thinking mainly about the availability of Remove Curse and Greater Restoration. I'd be happy with level 20, but it was mentioned that it's so unbalanced op then, that having more spellslots or more accessible spells that are only available up to level 12 might be a good compromise. I mean offensive spells don't scale to level 20.... This is probably what Larian wanted to get around and enable otherwise? I would also be happy with just Greater Restoration Scrolls like in BG 2, but BG 2 had also Exhaustion...