In OD&D, all magical armor is plate armor. Using only the LBBs, this isn’t so much an issue as 2/3 classes can wear magical plate with no restrictions, and the class that can’t, cant wear any armor anyway.
But the thief poses an issue, as that class can wear some limited armor, but not the better armors, and no magical armor.
Meaning that the Thief is practically just as bad as the MU. Worse, actually, as higher level MUs can make up for this deficiency where Thieves never can.