My rule on buying a MM would be that I'd want to use (but maybe never get the chance to) at least 30% of the content.
Only the MM1 exceeds that standard. For the others, its not even close - maybe 5% or less.
With monsters, I'm looking for an idea behind the design in which at least one of the following is true:
1) Ties into some strong instinctual fear.
2) Ties into some cultural myth or universal mythic concept.
3) Has an interesting and unique special attack or ability.
Beyond that, I'm looking for good presentation of the idea.
a) Strong, non-cheesy art, to sell me on the concept and provide a ready game aid.
b) Flexibility in the concept, meaning I can use the in a variaty of settings and in a variaty of ways. This favors monsters in a mid-range of CR that I can scale up by advancing them, or low CR monsters that make good swarms. For intelligent monsters, it favors things with strong coherent motivations so that either they suggest thier own stories, or else they can be used as complex NPC's which can be either allies or foes. It also favors to some extent monsters which have low ecological requirements (undead, constructs, outsiders, elementals) since you can put those just about anywhere, and which if living can adapt to a variaty of habitats. Something which has a flavor that only fits say tropical savannahs or arctic ice flows have a low probability of being used.
c) Good mechanics.
If you could buy monster manuals by the page, I might buy a few monster manuals in peices. But I'm not going to buy a $30 book in order to get access to 5-6 monsters that I could fudge myself from the concept alone.