Usually by the third monster book, the accusation of “cash grab” is often heard. Me, I don’t mind so much. I fondly remember 2e’s binder approach with their Monster Manual. You could buy different packs of monsters which were often organized by the campaign settings: Ravenloft, Dark Sun, Greyhawk, etc. If you were running a straightforward fantasy setting, then you knew you weren’t missing out by passing up the Ravenloft set.
With Paizo, their Bestiary series is starting to have a stronger emphasis on certain types. For Three, it’s the Orient with entries on Oni, Imperial Dragons, terra-cotta warriors and the infamous “jumping vampire.” I also like these volumes because they often incorporate new monsters that were created in previous modules and Adventure Paths.
Keep in mind I’m a completist, which colors my vote on making Three a vital addition for PF1 GMs. For the budget-conscious, it is available in the new, smaller pocket edition.