I'll add some perspective from the opposing side of the argument.
What is the official stance?
That depends on how you read the text included in each errata. While there are several versions of this errata blurb, each states the "primacy" rule,
When you find a disagreement between two D&D® rules
sources, unless an official errata file says otherwise, the
primary source is correct.
But, you must continue reading, as while there are different versions, nearly every version gives an example, like
One example of a
primary/secondary source is text taking precedence over
a table entry
For example, the table for Armor Class Modifiers under Favorable and Unfavorable Conditions has a footnote that appears on Helpless and Pinned, but the footnote mentions penalties that don't apply to pinned.
Treat the defender’s Dexterity as 0 (-5 modifier). Rogues can sneak attack helpless or pinned defenders.
I suspect the intent was to tag the latter portion for both, but it reads as if the penalty applies to both. However, the associated text for the two conditions is clear that that penalty (Dexterity as 0) only applies to helpless. (Different penalties apply for pinned that the table does not mention. The penalties are close enough in practice it will make little difference, unless by a misreading you allow a pinned opponent to be subject to coup de grace.)
That is what the errata blurb is intended to clear up.
If you read the primacy rule as establishing a legal channel, a problem arises when an intended change is a contradiction. The blurb is an acknowledgement that they make mistakes. But intentionally expanding or changing rules shouldn't be viewed as contradictory or illegal. Nearly every publication since the core rules has expanded or changed the rules in some way. Otherwise no publication after the core rules is a valid rules source. Since there are contradictions in the core rules, they wouldn't be either by this interpretation.
Also, reading the primacy rule in such a manner also means that sources such as FAQ, Rules of the Game articles or other materials from authors and designers are viewed as invalid in their entirety.
What's particularly provocative about the Rules Compendium is that it states up front that it supersedes previous rules and this is what people with a very constitutional reading of the errata blurb have a problem with.
What I find bizarre is that the Rules Compendium is not a new source of rules, rather a compilation of tidbits of rules changes and expansions that, in some cases occur within the Player's Handbook itself like the rules for charging versus the rules for Balancing that expand on charging and other times within other expansion books like the expanded rules for hiding from Complete Adventurer Expanded Skill Descriptions, pp 101-102. They are just presented in one location in the RC, so you don't have to look to the rules in Magic Overview and Casting in Combat and Actions in Combat to determine that unarmed "armed attack" qualifies for attacks of opportunity or heft around all your splat books to derive modifiers.
Yet the Rules Compendium is singled out as a uniquely gross violation of the errata blurb but the very sources it pulls from are not. It seems inconsistent for someone choosing this strict application of a footnote, when the principles in question have already been violated. The RC is nothing new, its just presented in one place, rather than as a small part of a larger work.
To address Hey I Can Chan's comments and post:
I think the disconnect here is that the same people who take the prevention channel on the errata blurb are also ones who don't "see" a rule unless it is spelled out explicitly. I'm not sure that manner or critical thinking has been common since before the Stoics, but for some reason it is prevalent in RPGs, especially 3.5 DnD.
Charging through Hindrances:
Most of this is right out of the PHB. What's new is stating that you can jump or use tumble during a charge, but otherwise the uses are already spelled out. I would argue there's nothing wrong with doing so before the RC, since both are ways to get around not having a clear path, or rather ways to have a clear path when they otherwise weren't already.
The Action Needed to Activate Some Magic Items
Not exactly novel, but valid nonetheless.
The Survival DC for Avoiding Quicksand: The original Survival skill check DC for avoiding quicksand is 8 (DMG 88). The Rules Compendium on page 103 increases this DC to 15 without commentary.
This is a valid point. Although, I'd hold of judgement of whether this was "new" without checking other sources, especially Sandstorm.
Touch Spells and Threatening an Area
This is right out of the PHB, but again, you have to put together the rules from at least three locations, Magic Overview, Actions in Combat and Casting Spells in Combat. One says you are armed, one says you don't provoke and the other says armed "unarmed" threatens. RC is just doing you a service here.
Using the Skill Sleight of Hand:
There's some new stuff here, unless it's from another source book. I don't think the dagger surprise is, but it wouldn't surprise me if it was.
What It Means to Be Hidden
Not sourced from RC, from Complete Adventurer Expanded Skill Uses. This is irrefutably my case in point.
When Daily-use Items Recharge
Not sure about this one.
Making the claim that the Rules Compendium is somehow different than other rulebooks released after the core, like the Complete Adventurer is what I find hard to understand. Perhaps it's just our collective lack of knowledge as to the exact source of each rule that clouds the issue. I'm sure there are completely new rulings in the RC, along with a great deal of restated rules, but it hardly makes it different than books before it.