Fanaelialae
Legend
This is a very tricky topic and I believe the designers need to tread carefully. Someone mentioned it in an earlier post about making a robe wearing mage viable. I think this is an important issue because if there is no reason for a mage to walk around without armour, then every mage build we see will be armoured.
Someone above also mentioned "well you can cast a 6th level spell". Well to be honest, what is the power difference between a 5th and 6th level spell? Think about casting that 5th level spell "and" attacking with your sword. Would that maybe equal the casting of a single 6th level spell?
I wouldn't want to be in the designer's shoes at the moment because I believe they have a lot of clean up work to do.
The difference is pretty significant. It's the difference between Cone of Cold (good damage) and Mass Suggestion (if it works, you can avoid the fight completely; that's not even touching upon uses out of combat). I would say a 5th level spell plus a sword swing is definitely not the equivalent of a 6th level spell (plus, you can't do both in the same round unless you use Action Surge, which is once per encounter).
The price of giving up spellcasting ability is not to be underestimated. Just because a Mage knows how to swing a sword, doesn't mean he'll be great at it. He won't get Extra Attacks like a Fighter would, or any of the other great fighter abilities. Additionally, he'll be green with envy when his non-dippy Mage buddies are slinging powerful spells that he'll have to wait another level or two to acquire.
You're effectively trading supreme, cosmic power for a bit more survivability. Given that arcane magic has some great ways for a mage to stay out of harms way (Invisibility, Fly) I don't think it's an easy trade at all. Except for dwarves, but even though D&D doesn't have a tradition of armored dwarven mages (heck, even the tradition of dwarven mages is a relatively recent one) I feel like it's appropriate.