I've posted some quotes relating to retainers from the Mentzer Red Box and Keep on the Borderlands in the comments of James M's post. I thought I'd deal with some from the AD&D PHB here. I'm most interested in hirelings since thats what the majority of my players have used.
"At any time a character may attempt to hire various different sorts of workers, servants, or guards." Well that pretty explicitly does not place any restrictions on the recruitment of hirelings. "Note that the number of hirelings is in no way limited by charisma...." That provides even less of a barrier than B/X and BECMI's undifferentiated "retainers."
The most interesting part, to me, comes from the Successful Adventures section. "Once an objective has been established, consider how well the party playing will suit the needs which it has engendered.... Will it be necessary to find mercenary non-player characters or hire men-at-arms in order to give the party the necessary muscle?" This quote makes it sound like the use of hirelings is at best a conditional state. If the party has a particularly tough monster it wants to go after in the dungeon, then you bring some mooks along. Otherwise you might have a linkboy or two, but if you're not specifically going into no man's land you're not going to be bringing a small warband with you.
Not sure what all this means to the greater conversation yet, but it's something to chew on.