Pinotage said:
You make a good point. But, imagine you had 500 'unique' creatures in a book and only 250 were useful, you'd still have 250 unique creatures to 'play' with and change to create even more creatures (like assigning each one, one of 6 roles). If you have 150 creatures, and 350 created from those, and only 75 of those base creatures were useful, you'd have a lot less creatures. I'd much rather have more monsters and the potential to create vastly more than a small number and no real potential to expand on that.
Pinotage
Well, your point sounds good in theory, but in practice it doesn't really work out as well as that. Namely, I think that the more a designer tries to make something "unique", and the more unique things that a designer has to create, the less useful and interesting the result.
In my opinion, there is something of a finite limit on how many truly excellent unique things (by your definition, for clarity's sake) can be created for any one monster book. This is strictly limited by factors like mythological history, previous good adaptations in D&D, and basic factors of how much time and energy the designers have to put into making interesting creatures. Because of these limits, I would not say that a book with 100 unique monsters and a book with 500 unique monsters would have the same ratio of good monsters to bad monsters. The book with 100 unique monsters may have 50% being good, but the book with 500 might only have something like 20% being good. If the book with 100 unique monsters then has something like 5 versions of each monster, we end up with two books that have 500 stats for creatures, with one being 50% good and the other being 20% good.
I guess I could phrase this as an idea that having multiple variations of a creature helps extend the advantage of a well-designed base creature. If a base creature is good, then people will want to build encounters, adventures, or even campaigns around that creature. In cases such as this, especially with monsters that have large populations and their own cultures, having a wide variety of versions of that creature makes it easier to build such adventures and encounters, making the DM less reliant on other unique base creatures (which may or may not be as interesting as the adventure-centric base creature).
Also, there is one more argument for the first Monster Manual in particular having a large number of "classed" monsters: the first 4E MM is going to be the first Monster Manual for new DMs. Having the kind of variety within a base creature type that the MM1 will have serves as a kind of tutorial for new DMs who don't realize the full extent to which the new monster deign scheme enables them to modify monsters and create new ones.