I was an RPG forum the other day and I came across a guy talking about how it was "unfair" that his DM didn't include every monster in the Monster Manual. My head very nearly exploded. His argument essentially boiled down to the fact that the Monster Manual is a Core Book and everything in the Core books must be included.
The ridiculousness of this is just staggering so really take a minute to let that one soak in.
Thankfully the fact that this particular player was taking this bullshit complaint to a message board meant that the DM did the right thing and rejected the players assertion that all monsters should be included. I'm going to go a step further and say that all monsters should NOT be included.
Depending on edition you are talking about hundreds of monsters, some as ridiculous as this guy.
That is a Giff. Yes, it is a Hippo-Man with a gun.
But aside from the really stupid monsters one can find in the core Monster book (to say nothing of the plethora that can be found in the books beyond) you have to deal with over-saturation, unnecessary additions, overlaps, and monsters that are just flat out of place in your campaign world.
Gamemaster often spend countless hours meticulously crafting a world from their imagination. They consider a whole host of factors often spanning thousands of years of history. Those who go to that level of effort do themselves a disservice by forcing every single monster in the core books just to appease a player. I'm certainly not saying that you couldn't craft a world that fit all of these monsters, but given creatures like the Giff and the Shedu (seen to the right) that is a tall order.
Monsters are an important part of the world that you as a Gamemaster build and run. Don't just throw one in on a whim and don't try to fit too many varieties in. Doing so can be counter effective in your quest to create a rich fantasy setting.