I have been known to say on occasion that the only "true" core books in 1E are the Players Handbook, the Dungeon Masters Guide and the Monster Manual. Everything beyond those three are not "core" and thus are accessorial only, Completely unnecessary.
Having said that. I must admit that I am a fan of the Monster Manual 2 (MM2. not necessarily because of the monsters listed in it. Truth be told, I think at good 50% or more of them are beyond stupid and I will never likely use in a game.
The supplemental information provided at the beginning and after the monster listings is just plain awesome. I love the Wandering Monster tables that are included. The "How To" make your own Wandering Monster table using the commonalities groupings and the subsequent customized tables of monsters provided for DM use in said tables.
Are those tables and information "necessary" to the game? Of course not, but they sure are handy as all get out.Anther great add in to the MM2 is the dice roll table at the beginning of the book. An example is how useful this table is would be that my kids are now beginning to try their hands at DMing games. As they immerse themselves into learning the varied mechanics of the DM, seeing the sometimes "weird" (as said by my 12 year old son who is DMing a game this weekend for us) die rolls called for had him stumped until I showed him this table in the MM2.
When he saw that a giant rat does 1-2 damage in a bite attack, he was wondering how he would roll 1d2? He didn't have one of those. That's when I broke the MM2 table out for him. (for those not in the know, the table says to roll 1d6 with 1-3 being a result of "1" and 4-6 being a result of "2".)
It really is the inclusion of tools such as these that make the MM2 worth the money spent on it. I could live without the monsters it added to the mix.