The problem, if you put it in core, is you run into the 4e design philosophy that everything in core is meant to be used. It's not like earlier editions where you got reams of material in core and it was highly unlikely that a given DM would use every single thing. DM's would pick and choose depending on what they wanted.
4e isn't built like that. It's a much more focused design. (Whether that's a good or bad thing, I don't know)
So, if you include something like this in the core design, then you have to allow for it in every subsequent book. Every module now has to take into account these rules. Do the players have Boots Low Soft, granting them a +1 Sneak? How does that affect Skill Challenges? Do the players have the resources of an entire kingdom at their disposal? How does that affect adventure design? Do the players even have the resources of a mid sized trading house (which can easily be tied up in thousands of gold pieces of value) and how does that interact with everything else.
Forgive me, but I'm still not seeing a problem: universal rules for things like Donkeyhorses and Quarterstaves (hmmm...the name of the next big RPG phenomenon, why not?) shouldn't negatively impact your game.
And a DM still doesn't have to include everything. Assuming, arguendo, that the 4Ed world works like the RW in terms of distribution of goods, some things may be available in some areas and utterly unavailable in others...or only at great expense. That's Econ 101.
A list of mundane items typically gives you their stats and their prices
assuming normal availability. A mountain community may not have access to a cheap 10' wooden pole since all they have in their vicinity is the fantasy world's equivalent of the gnarled Bristlecone Pine.
As for "Everything is Core,"- I can guarantee you that that mantra is, essentially, meaningless.
I play with a bunch of guys who generally design their campaign worlds to be fairly restrictive. We're just now playing in a 3.5 game that allows the use of any of the Complete series...and then, only the first 4.
In the past, some of them have outlawed Monks, Assassins, Paladins and certain races from their 1Ed or 2Ed games.
Nothing,
absolutely nothing, will prevent a DM from excising material from his 4Ed game that he doesn't like- Tieflings, Psionics, whatever. It won't unbalance the game or otherwise make it unplayable.
"But what about what happens when an adventure designer assumes the party has a psionicist of some kind?"
I've seen this kind of thing happen in every previous edition of D&D, and my answer is the same: then that adventure designer has made a mistake. You should never assume that a party has a particular ability at its disposal. By doing so, you create a bottleneck that some parties will not be able to pass. A good designer should make sure that there are either multiple ways to get past a bottleneck, or supply the method of success within the adventure's narrative.
And if you can excise classes and races, you could just wing it and excise Donkeyhorses as well.
And, how do you balance a game where half the players are buying inns and businesses and the other half are buying adventuring equipment?
4Ed in no way abrogates the duties and powers of the DM, and that's a DM issue. In the past, when this was raised in games, other DMs have essentially said that you either have time to run your inn or go on adventures, you can't do both. At best, you can have someone run it for you while you go gallavanting around the countryside bitchsmacking dragons.
And in that, 4Ed is no different from 1Ed or 2Ed with their "stronghold" rules. Yes, your Fighter just made it to the level when he can build his stronghold...but does he have the money to do so? Does he have someone he trusts to run it while he's away?
What happens when he returns to find a Usurper on
his throne? (
GRRRRRR!)