At what point do house rules become their own rule set? I think it's an important question, and one that deserves a conversation larger than just one blog (no matter how excellent I think said blog might be). It's a question that points to the heart of our hobby, and hearkens to the earliest days of the hobby, in terms of how the game was actually played, and how people who played it thought of it.
At Metatopia this weekend I made the point that, back in the earliest days of the hobby (1974-1979 or so), one person would have a copy of the LBBs, an issue of the Strategic Review, a home-brewed magic system based on Piers Anthony's Xanth books, and the Ready Ref Sheets from Judge's Guild, while someone else might have a photocopy of Men & Magic, a few issues of Alarums & Excursions, and a bunch of house ruled combat tables based on his years as a white belt in the SCA.
Today, we would say they were playing completely different games, if one compared the mechanics.
And yet back then they were both called "playing D&D". You'd take a character from one campaign and play it in another without any thought, converting stuff on the fly. The definition of "the rules" was flexible enough to accommodate house rules. You didn't need a different name for what you played. It was all D&D, even if they were nigh unto unrecognizable from table to table.
We took the "don't let us do the imagining for you" imprecation seriously. "D&D" was less a collection of specific rules and a general style of play - an aesthetic.
I can't help but think that we're seeing something similar in today's OSR. Not outside the OSR, I should add - someone playing Dogs in the Vineyard isn't likely to bring in the conflict resolution system from Shock, the shared narrative mechanic from Donjon, and say "we're playing Dogs in the Vineyard tonight." The silos are too well-defined, and modern games are too dependent on rule mechanics rather than the aesthetics of play.
I must wonder if the fragmentation that some lament in the OSR isn't actually a hearkening back to the earliest old-school aesthetic. I don't think the labels really detract from that aesthetic, because they don't really define walls so much as sources. Picture a game played using the Labyrinth Lord rules with the mountebank and jester classes from Adventures Dark and Deep, plus the encumbrance system from Lamentations of the Flame Princess, and a couple of printed-out issues of Crawl.
You're still playing D&D.