The 5 basic problems with Rules Compendium:
Now, don't get me wrong; I actually like some of the changes in Rules Compendium; in particular, making the activation time for every item which produces a spell effect be the same as the casting time for that spell makes a lot of sense to me. But the way the authors of this book went about changing the rules — essentially introducing a "stealth" 3.75 release — was just a mashup of arrogance and fatuousness. Consequently, every change I adopt from Rules Compendium is labeled a house rule.
- It's billed as a compendium of existing rules (which it largely is), but also throws in a bunch of new and changed rules, contrary to the WotC press releases about the book's scope.
- Its authority is entirely self-declared. I haven't found any statements (web site announcements, interviews with the authors, or printed announcements) by WotC that in any way backs that claim to primacy, and I've done quite a bit of looking.
- Rules Compendium only claims precedence over preexisting books (as Flickerdart mentioned).When a preexisting core book or supplement differs with the rules herein, Rules Compendium is meant to take precedence.
- The new (2013) Premium 3.5 core books don't contain a single one of the changes introduced in Rules Compendium. They do contain changes that weren't in the combination of original books + latest errata files. These are new books, which Rules Compendium even in its self-appointed dominance position has no say about. Without specifically saying that's what they were doing, Wizards of the Coast has officially backed out every change introduced in RC.
- Again as mentioned, the Primary Sources Errata Rule says the three core rulebooks are primary sources, and only official errata files are allowed to override them. By this rule (repeated many times by WotC in the course of 3.5 errata files) Rules Compendium is never allowed to win any disputes with the primary sources.
The official WotC SRD is mostly the original core rules, trimmed (sometimes butchered) to remove examples and other "product identity" trademarked content. It doesn't include most of the changes in the freely available errata files. The most convenient "SRD" site, though (d20srd.org) does combine lots of the freely distributed D&D content to make a linked, searchable end product. This content is not superseded by Rules Compendium's claim of primacy, either, because none of it is in the form of "a core book or supplement".
That's the situation from a RAW perspective; make of it what you will.