There are lots of elements of old school play that have been forgotten over the years and one of the ones I miss a lot is rival adventurers. Starting with OD&D, whose random encounter tables included entries for "Veterans" and "Mediums," and continuing at least through 1e -- I'm not sure if the tradition was continued in 2e -- there was always the implicit assumption that the player characters weren't the only adventurers delving into a particular dungeon. These other adventurers might not necessarily be evil, but, seeing as they were likely after the same things as the PCs, they could certainly be called rivals, with whom the PCs might even come to blows, as famously illustrated in the Dungeon Masters Guide, which describes the battle between Aggro the Axe, Abner, Arkayn, and Arlanni against Gutboy Barrelhouse, Balto, Blastum, and Barjin.
Rival adventurers are useful in a campaign for a variety of reasons. Firstly, they provide a link between the PCs and the outside world, especially in a megadungeon campaign. They're as much a part of Gygaxian naturalism as are monsters who do more than wait around to be killed. Second, they serve as a useful goad to the PCs. If they know they have rivals who are after the same goals as they, odds are good they might move a lot more precipitously, thereby leading to some interesting situations. In my Dwimmermount campaign, the players have found signs that someone has been in the dungeon while they were away healing and disposing of their loot, which has added another layer of urgency to their explorations. Finally, as a referee, rival adventurers provide an opportunity to roleplay with more depth than one is typically afforded by most monsters. I love playing the role of venal, self-interested antagonists; it's fun in a way that playing Pig-Face Orc #231 is not.
I suspect that one of the reasons why the idea of rival adventurers dropped out of gaming is because of the time consuming nature of creating them as rules become more complex. In ancient times, I could make up random adventuring parties on the fly by just referring to a couple of tables and making some quick dice rolls. I can't even begin to imagine doing that under 3e, for example, and I suspect I'm not alone in feeling this way. That's a shame, since rival adventurers bring a lot to the table and greatly expand the scope of the campaign in small but significant ways. That's why I was glad to see the concept included in Castle Zagyg: The Upper Works. I'll be including several such groups for use with my megadungeon project and I'll be using them with Dwimmermount as well. It'd be nice to see other old school projects do the same.