Touch range on spells, I think, feels too clunky, and robs melee of the utility that they can otherwise provide to the group (unless you want to micromanage the toggle, which seems like it would be extremely irritating).
What if the toggle instead increased the cooldown on many (most? all?) spells and spell-like abilities? That way, it's not attractive to pure casters, but melee characters that want to be able to throw out a heal or a buff here and there are not significantly impacted.
Or maybe if casting spells while the toggle was on applied a stacking debuff that increased SP costs? It wouldn't need to have a long duration or a massive effect, but even like 5% per stack, 10 second duration would probably be enough to heavily discourage casters from using it.