I own eighteen different RPGs: D&D 3.0, The Fantasy Trip, DragonQuest, GURPS, Vampire, BESM, Hero System, In Nomine, Passion Play, Star Wars (d20), Chivalry & Sorcery (3rd edition), James Bond 007, Fuzion, CyberPunk 2020, Bushido, Call of Cthulhu (Chaosium version), Hero Wars, and ForeSight.
Among the eighteen of them, all put together, I own one monster book: the 3.0 MM. It is more than enough: in fact I almost never open it. People make far more interesting opponents for PCs than monsters do.
IMHO. YMMV. YDWYDWP.
[edited to add] By the way: I note that this poll doesn't allow for the person who owns no monster books. Outside monster-obsessed D&D that is not unusual, and within the D&D community there are non-GMing players who do not own the MM or DMG.