Flavor

IIRC there's not too many spells that are the same between the OG D&D classes.

You really need a cleric and a magic user to have all the spells you need.

GURPS Dungeon Fantasy clerics and druids use the same spell lists as mages, so a mage could have all the spells needed.

It's a flavor thing.

The big difference is clerical spells will work in zero mana if the sanctity of the location is high enough.

The character will be constructed differently too.

The rules as written don't forbid a mage from wearing armor; but I have applied a penalty to casting based on encumbrance before.  I've also applied a penalty based on the amount of iron the mage is carrying. 

The goal is to capture the feel of AD&D without rewriting the entire system. 

I've even gone with social restrictions.

There's no fundamental reason for a mage to not wear armor, the Wizard's Guild and Warrior's Guilds have an agreement.  If a mage is a member in good standing of BOTH guilds, then they can wear armor...

I need a skill list for the minimum entry level guild membership for that; but that's not new rules.  That's world design!