I'm with Paul on this one. There are times when less is more.
I much prefer the current 5E single Monster Manual, followed by occasional books like VGtM and MToF that provide much more depth to the creatures in the game, than endless reams of uninspired stat blocks for increasingly strange and improbable creatures.
I have hard copies of the 3.5 Monster Manual 1 through 4, plus Fiend Folio, Lords of Madness, Libris Mortis, the Draconomicon, Drow of the Underdark, and the two fiendish codices. I sold off many of my third party 3.0/3.5 monster books, except for my Ravenloft and Midnight collection. I seldom used anything beyond the original Monster Manual or setting specific books in game, but really enjoyed seeking inspiration/lore in the more focused books. And of course, I have just as many 1E and 2E monster books.
Same goes for PFRPG 1.0 - I have hard copies of Bestiaries 1-4, FGG's Tome of Horrors Complete and ToH 4, Green Ronin's Advanced Bestiary, and others I'm probably forgetting.
I currently prefer 5E, but consider new monster books (whether WotC or 3rd party) low on my priority list. While this may partly be due to the fact that I don't GM or even play nearly as often as I would like to, ultimately, I prefer quality to quantity.