In running Dwimmermount, I've made use of material from all the OD&D rulebooks, including all the supplements. This is in keeping with my "D&D 0.75" approach to the game -- more complex than a pure White Box campaign but not full bore proto-AD&D either. So, if you play in my Dwimmermount campaign, you'll find magic items, character classes, and spells from sources other than the LBBs, though, in most cases, I use them as "spice" rather than as the "main course," if that makes sense. The one area where I haven't made extensive use of the supplements is monsters. In general, I've stuck primarily to the monsters found in Volume 2 of OD&D, supplemented by beasties of my own creation. The reason for this is simple: I find most of the monsters introduced in the supplements too distinctive for use in my campaign. By that, I mean that most of them feel too strongly associated with D&D.
Now, that probably sounds like crazy talk and perhaps it is. After all, I am playing D&D; why wouldn't I want monsters that are strongly associated with D&D in my campaign? Well, the game rules I'm using may be D&D but I don't think of the world I'm describing through them to be a "D&D world." To me, it's merely a fantasy world -- a fine distinction, maybe, but a real one. To put it in more concrete terms, nothing about the presence of, say, a goblin or even an orc in an adventure suggests anything about the nature of the world outside that adventure. But a beholder? An umber hulk? A sahuagin? Demogorgon and Orcus? Monsters like those carry with them certain larger assumptions, or at least expectations, that I don't want brought into my campaign.
Obviously, there are degrees here and everyone will personally draw their lines on different places in the sand. For example, I don't have any problem with carrion crawlers or displacer beasts in my Dwimmermount campaign, even though both of these creatures are rightly, in the minds of many, seen as "D&D monsters." So, I can imagine someone feeling similarly about beholders or ixitxachitl, while I reject them both as so strongly tied to a certain specific conception of D&D that I don't want to bring to bear in Dwimmermount. If I were playing a Greyhawk-based campaign, though, I wouldn't hesitate to use most of these creature, because, to me, they're part and parcel of Oerth.
At the same time, I want to go on record as disliking the approach adopted most famously by Lamentations of the Flame Princess wherein every monster is unique. To me, that'd be like playing AD&D using Appendix D of the DMG in place of the Monster Manual. Certainly it'd make it harder for players to use their much-hated metagame knowledge to their characters' advantage, but I've come to believe that, the more of D&D's fundamental building blocks one rejects, the less the game feels like something recognizable to anyone outside that campaign -- and monsters are a very important building block. Which ones we use (or don't) can have wide-ranging consequences.