Well, you can be a lot, but we still need rules and regulation, it is important for balance. Higher/tougher armour makes you tankier, so you shouldn't be able to burst as well. Also, it makes sense from an RP standpoint of things, but that is a different topic.
Also, you can have medium armour and still a full set of spells at your disposal, I believe you just haven't found them.
Also I have a battle mage set as a gladiator. One for damage and one for CC. I think you just didn't spend enough time yet to see what you got at you disposal.
And again, limits are needed cause rules are necessary.