Your experience with Magic-Users matches my own, except in my campaigns people stopped playing single-classed MUs fairly early on. For most of my years playing D&D, the only MUs were multi-classed characters.
The whole thing with metal armor never bothered me. If you don't allow multi-classed MUs to wear metal armor, then it seems like one could incapacitate an MU by putting him in a metal cage, or forcing him to wear chainmail. You could even have a small room with stone facade walls over iron to trap a powerful MU. I think the concept is pretty interesting, but in the past it's seemed like more trouble to work out all the ramifications than it was worth. Thinking about it now, it seems kinda cool and maybe I should add it to the supplement.
Fair warning: If you insist on the class making sense, you'll get expelled from the Old School
I've got the completed second version out in the Project Feedback thread. It's updated from the one I posted above.
