I like and try to apply most of Celebrim's suggestions in my campaigns. Some points:
1. Less variety can be more variety. When two dozen evil humanoid races abound and, worse yet, regularly form coalitions to overrun civilization, nobody's going to tell the Gnolls apart from the Bugbears anymore, or the Tasloi from the Goblins, or the Shadar-Kai from the Krinth. DMing the Red Hand of Doom, I decided that Goblinoids are the only form of evil humanoid life out there, and worked on a way to explain how Goblins, Hobgoblins, and Bugbears are actually just different phenotypical expressions of the same genotype (gender, but more complicated). That way, the goblinoid invasion had much more the feel of The Other coming the get Us, or Away Team attacking Home Team. Us vs. Them is a powerful theme that shouldn't be diluted.
Moreover, it's much easier to make a single monster race's psychology and outlook unique and interesting. That leads to more interesting character play. Some NPCs that worked with the Red Hand were actually memorable villains for more reasons than giving the PCs a good fight.
2. Theme is important. In my current stone age-y, low magic, E6 campaign, basic animals are powerful threats. And I intend them to stay such. A Ranger with FE: Animals is actually an important asset, because in this world, 'normal' wildlife is
dangerous. I'm trying to give the campaign a realistic, gritty feel, with just occasional supernatural occurrences. I even limited class choices for players (no written language --> no wizards; no formal religion/church --> no clerics). So "a wizard did it" isn't likely to be cutting it when it comes to explaining strange monsters.
I do include lots of animals, plants, vermin (oh! the giant vermin in this world...), a few oozes, some magical beasts, and some undead. Constructs, elementals, outsiders, giants aren't part of this world, though. Fey and dragons strike me as too medieval-european, as well. There are no humanoid races except for humans (and a high-tech, hyper-evolved human variant). The monster selection is trying to keep to my basic theme, and so far, my players don't show boredom, but fascination. When was the last time your players were afraid to enter a cave because there might be bats? We just had a blast fighting some!
3. You can always reskin and retool stuff. For example, I wanted a 10 foot long, slimy, translucent worm, thin, but very strong, that wrapped around people and strangulated them. I used the functionally equivalent stats of a giant constrictor serpent, but my description gave my players more of a creep. I wanted a stinking, disgusting lizard-like predator. I just used Troglodyte stats, sans weapons and with Int 2. I wanted a giant crab, but instead of
this unbalanced monstrosity I just ended up using a giant scorpion with no sting attack.
You can also get a lot of variety out of just adding or subtracting a single ability. Water-dwelling spiders? Done that, just add the aquatic subtype and a swim speed. Snakes with wings? Really simple, and cool. Giant, mutated, landbound bat monster thing? Desmodu War Bat w/o fly speed, albino for flavor. Ranger wants a really unique animal companion? Take a Displacer Beast, adjust stats for small size, take away tentacles, add pounce and rake, call it a freak of nature (that one took a little more work, but still).
4. Lots of different, but similar stuff can have an impact. When the PCs invaded the stronghold of a mass murderer with a snake fetish, I really enjoyed the masses and masses of serpent-themed monsters out there. The BBEG had a magical artifact that attracted serpentine animals of all kinds, so there was everything from vipers to dung snakes to sewerms to constrictors to jaculis... the artifact was guarded by a Naga that was bound to it. The BBEG had slaughtered dozens of the snakes that had come to him, and used the artifact's powers to create a Serpentflesh Golem to serve him. All this made for great theme, but included a
lot of mechanical variety, so the battles never became boring.
You see, some of the monsters found in the books will probably never be used. But there might also come a moment when you want to use a dozen different creatures of a similar theme, and be thankful you bought Serpent Kingdoms and the Fiend Folio (for example).
All in all, I'd say having options is great. You never know when you will stumble across a monster you never used and just think "Damn! This is exactly what would fit for next session!". If you dislike certain monsters for their flavor, simply use the statblock for something of a different description, that you come up with yourself. If a certain monsters has cool flavor, but lacks a certain something, it's often easy to just add a template, or even a single ability (e.g., a movement or sensory mode) and make it work. And if you want to limit yourself in what you use, fine. You don't have to make use of every monster in the books. But you
can.
