This is something that does potentially concern me - early 4e design notes make clear the designers were indeed looking to strip out stuff like monster stat blocks for creatures the PCs wouldn't be expected to fight.
So I just tested it by checking for Unicorns and Pegasi in the Rules Compendium. Unicorns are right there in Monster Manual 1, so that's ok. Pegasi don't appear as a monster, but they were statted out as a DMG2 style Companion for PCs in a Dragon magazine article.
I still think it's a problem when adventures don't give me stat blocks for important non-hostile NPCs. Maybe I don't need combat routines for them, but Level, Attributes, Skills, and a paragraph of proper roleplay notes (as in the Loudwater section of FRCG, but missing in most of the published adventures, where most friendly NPCs are little more than names) would sure help a lot.
Regardless of edition, I think it's reasonable that some things are going to be left out either because testing says they don't see much demand, or creating them would just be too unwieldy. But having stat-blocks is a two-way street and has significant ups and downs.
First: having stat blocks can be good for the DM because
if your players decide to engage the creature, you've got the info handy, yay!
But: some NPCs are just window dressing. You're in a magical forest, oh look there's a pegicorn! It's purpose is just to add to how magical the forest is, if the players came one step closer it would hurry off into the skies/woods.
At the same time: it can be a lot of information to handle. Sure, it's nice to be able to grab a stat-block if it's necessary, but sometimes that lvl1 Farmer NPC is just so pathetic it's really not worth the effort in rolling over him when you're just going to roll over him.
And of course: players have no idea if a creature has a stat-block or not, and IMO they shouldn't. I create a lot of home-brewed NPCs in my 4e games, so even if my players did see X monster off in the woods, there's no guarantee it really is
that monster instead of my own that's similar, which means no guarantee of a stat-block or not.
Worst of all: premade stat-blocks for everything can be severely limiting. Why is a Wraith a CR7? Why is a banshee a CR14? Why a Drider a CR9(or 8, or 6 depending on your books/system), without a tool to alter or very clear math on improving or downgrading monsters, it can be difficult to find good challenges for players without being stuck with a limited selection of foes.
--Addendum: this was one thing I liked about some of the monster classifications in 4e, because it gave you a generic stat-block to work with based on the style of combat you wanted the foe to fight.
--As a side note: What I consider to be the
worst aspect about "stat blocks for everything" is just that WOTCs realization of how a monster fights, how tough it is, what powers it has, may not jive with my realization, and as above, without a tool or obvious math for altering this, it can be severely limiting or just downright harmful to your enjoyment of a game.
I find that certain mechanics can interfere with my abillity to do in character RP the way I m accustomed to. I also find that while D&D is a far cry from a realistic RPG, 4th edition chalenged my suspension of disbeleif more than any other.
Fair enough, though I will argue that there are lots of ways to RP and I think it's reasonable that a game might encourage people to branch out.