Ok so I'm letting magic users cast spells in full plate. There I said it. Now fire at will!
Actually to be honest what I'm doing is not explicitly prohibiting magic users from casting in full plate, I'm just making it cost prohibitive and quite unproductive. See with the ideas coming out of the fatigue rules it becomes very hard for a magic user to be a good magic user and cast in full plate.
Face it, high attributes are a scarce resource. So it would be uncommon to have high intelligence, high strength and high endurance so your character can be both proficient in spell use and able to carry the burden of spell casting AND wearing full plate.
Intelligence gives you a rating of how well you can channel magical power, a term called flux (and the attribute is actually illustration, not Int). The higher the attribute the higher the flux and the more powerful the spell caster is. Strength allows you to carry more. With lower strength the armor feels heavier. On top of this endurance dictates how much fast paced activity you can do per round and as you might have guessed spell casting is one of those highly demanding activities. So having a medium to low endurance and performing one of the most demanding activities which is spell casting, and do so inside a heavy piece of armor could be too much for the wizard.
Sure the wizard will get a spell or two out, but will be so fatigued afterwards it won't be cost effective. If the party gets rushed the wizard might be too fatigued to effectively run away. Overall it isn't worth the trouble to wear full plate.
Other articles to read :
Character Fatigue - A preamble
Character Fatigue - Movement & Encumbrance
Character Fatigue - Round to round fatigue