I agree and wish to say a bit more:
The basic forty or so monsters in the DFRPG boxed set are great. They cover a broad range of archetypes (vampires, regenerating trolls, dragons, werewolves, etc.), but there's a whole separate tactical dimension on top of that: I'm still learning new tactics even just for the three varieties of giant spider on one page. (Currently I'm considering differentiating my spiders not by power but by NASTINESS, with the deepest levels of the dungeon having spiders with the same stats as up top but the nastiest tactics I have discovered.)
You could have dozens of adventures with just half the monsters in the boxed set and never get bored. The way I see it, more monster books are helpful not so much just to add variety as to (1) spark ideas, (2) ease conversions from mythology or other systems. You can make up your own hydra or Nemean Lion using the traits in DF Monsters, but it's kind of nice to have a book version already there. You can make up a mummy stat block in order to run Barrowmaze (famous OSR dungeon crawl) but it's nice to just have one ready so you can focus on running the adventure--and maybe the author will have thought of some nice twists that you would not have invented yourself.
I do have one complaint with DFRPG monsters, but it's one that applies to D&D 5E too: their writeups are pretty combat-oriented. They don't tend to have things like the Habitat and Ecology sections from AD&D 2E monster manuals, and they don't have diet/number appearing/frequency or even Treasure Type either. My solution has been to rip off Rasputin's spreadsheet (
Happy Thanksgiving! Here is the monsters and treasure list), which in turn are ripping off the ACKS notion of treasure strategies (explained here:
Updated Dungeon Fantasy Naturalism). So far that keeps me happy.
P.S. Oh yes, and it can be hard especially for a new GM to eyeball the power level of a monster, especially since as mentioned above it depends on how nasty your tactics are. 5E CRs are a terrible measure of power, but DFRPG doesn't even have CR-equivalents (except on Rasputin's spreadsheet, as "CER"), so at first you're not even sure which monsters are supposed to be horde monsters and which are small-group elites. Run a test combat to get the feel, but bear in mind that you might be overlooking a nasty tactic (or a rule) that the monster much stronger or weaker in actual play than they were in your test combat. The only actually useful guideline I can point to is Lanchester's Square Law: three Grues is about twice as nasty as two Grues, no matter what the stats are for a Grue.