Oblivion was a great game and all, but becoming the Arch mage without ever once casting a spell, is just bull, plain and simple. Morrowind had it so if you wanted to get to a high rank, you needed to know what you were doing in order to achieve it.
But it wasn't as if Morrowind required you to cast spells either. You could pay the trainer to increase your magical skill to get yourself a new rank. In Oblivion at least there was a quest that required spellcasting, though the brawn types could still complete it because they had spell scrolls. If we actually want some role playing to be involved in the guilds, they need to make the quests somehow require the skillset they promote. In Daggerfall, they asked you to cast specific spells to complete quests, I don't think that's unreasonable.
And that works out to the same effect, without a fake barrier in the guild. How are they supposed to measure the difference between someone of 69 skill and someone of 70? We can't treat the skill numbers like they are real in the gameworld, we have to treat the skill that they represent as a real thing.