I'd rather have some medium number of monsters with stat blocks for several variants than 500 distinct monsters with unpronounceable names, weak or non-existent ties to lore, and overall concepts that amount to "It's another fish-dude". Today's special monster the Xxgr'lkg is brought to you by the letters X and G, and their friend the apostrophe!
It seems that many of the bizarre and seldom-used creatures in previous incarnations of D&D were created out of thin air and bad design as part of a monster arms race for increasingly knowledgeable players. The GM needs to throw stuff at the players that will be challenging, but they've already memorized the stats for the current MM and as soon as you describe the critter they say "Ahah, I know that one, it's weak against fire spells and has a grapple attack with 10' reach". So you have to make up new stuff that they haven't heard of yet.
The CR system also dictated that you needed hosts of different underground-dwelling evil humanoids for them to fight, but that really had the same flavor (plain vanilla). Orcs, kobolds, goblins, hobgoblins, troglodytes, gnolls, bugbears. What's the difference in these again? How do they feel different in play? They don't much. /yawn
If 4E has managed to make fighting kobolds different from fighting hobgoblins (aside from how easy they are to kill), then kudos to WotC for pulling it off. If there isn't any flavor text in the MM, I'll be disappointed. I do want some idea of what makes each monster unique, where they live, and some hint of why they are bad and need to be killed by adventurers.
I think having several different ready to use roles in the MM helps make an encounter with 5 orcs much more interesting and believable than if they're all the same. Creative GM's with a lot of time would go through and give class levels and feats to those orcs to make them different in 3.5, but many folks lacked the time or desire to spend that much effort on prep, and so used them out of the book in their boring plain-vanilla version. The new MM will come with strawberry, chocolate, rocky road, and mint chocolate chip flavors of monsters. It sounds like some people are concerned that plain vanilla won't be represented (no base orc for example). That's a valid concern if you want a base on which to add templates, but it's not one that I'm especially worried about. I think we'll be able to customize from whatever variant is closest to your goal. Want a super spell caster? Work up from the base caster type. Rogue? Work from one of the archer types for the similar stats.
I'm happy with the idea of having some different roles for each monster statted out in the books. There's no need to use the names if you don't like them, just describe what the players see. The players will distinguish between them based on what the monsters are doing or how they are equipped (for humanoids). "I'll shoot the one with the crossbow". They don't care if it's called a "sharpshooter". They also can't necessarily tell just by looking at them how powerful they are and what their exact role is. I like it.