Let me just present this scenario: You have a 12 year old nephew learning to DM. His friends are playing and their party comes upon a hostile party of npcs. The player mage casts charm person on the npcs. What is the npc's save?
Assuming everyone's human, for Swords & Wizardry you have one question to answer: is the targeted npc a non-wizard (15 to save) or a wizard (15 +2).
I did that from memory. Do you remember the Labyrinth Lord saves? Here's a refresher:
So, for the 3 core classes, you have three answers and different answers still, if the charm person is coming from a wand.
You want to play that way because it seems more verisimilitudinous to you, cool. But don't kid yourself that you aren't paying a price in simplicity and playability. With Labyrinth Lord I guarantee your nephew is going to have to stop play and check the book. Hell, I would.
I know this sounds awfully like me saying James is "doing it wrong." If James want to play that way fine, but he's more than just a DM he's got hundreds of people following his blog. It's frightening to me that we have this opportunity for a fresh look at D&D with these retroclones and the main tendency seems to be to figure out how to slap all the old cruft back on them.
It's frightening, because if everyone starts playing it that way, that will be my only choice of campaign to play in (the way my only campaign to be a player in now is 4e, because that's the dominant form of play).
So, do you really need to turn S&W and LL into AD&D? Why not use Osric? Or, why not just play AD&D?