Under the BX rules anyone can play one of the four human classes. No matter how poorly you roll, you can still play a Fighter, Magic-User, Cleric or Thief. AD&D has minimum stat scores necessary to get into these classes, which meant you couldn't play a clutzy thief or a foolish cleric. Being a big fan of characters who suck, I'm all for allowing PCs to be terrible at their jobs.
The one place this great idea hits a speedbump is for magic-users. Dig this chart:
You could squeak by on an MU with an Intelligence of 4 or 5, as spellbooks aren't necessarily written in Common. But an Int 3 magic-user can't read his or her spellbook and has trouble pronouncing the words of the spell correctly. Maybe the rare Int 3 MU is an idiot-savant, barely functional except for a natural talent at reading and speaking Magicese.