I find Rob's proposal and reasons for adding race frequency to the game very...odd, to say the least, especially with 5th Edition's purported goal of unifying all the editions. Tagging races as common, uncommon, rare, or whatever does nothing to inform the DM how these might fit into her campaign. It just sets a bar. A bar whose only purpose seems to be passive-aggressively enforcing someone's idea of what races we "should" be using.
What makes this proposed mechanic even more bizarre is that 2nd Edition saw the introduction of numerous campaign settings that included more exotic races, such as: half-devils, half-angels, half-elementals, half-organic walking shapes, bug-people, half-dragons, bird people, hippo-men, and more. This begs the question, if tieflings are rare, but I am running Planescape, do I tell players that the common and uncommons are okay, plus tieflings? I guess I could just say common, uncommon, and rares are all good...but then what about warforged? Kalashtar?
And then there are the DMs that create their own campaigns. What if I create a world where tieflings and dragonborn are the most common races to be found, doing that whole war between Bael Turath and Arkhosia bit? What about a campaign that is largely restricted to mountains, with dwarves, minotaurs, genasi, and the odd warforged here and there (built by dwarf artificers I guess)?
I get that Rob grew up being exposed to certain media, and so prefers his game one way. Other players did not, or do not like the same things. Arbitrarily labeling races that he was "shocked" to see as being in 4th Edition's first Player's Handbook--while still ignoring the gnome write up in Monster Manual, I might add--as rare seems like he is both pushing an agenda and making assumptions about the game world, as opposed to the 5th Edition mantra of giving us the tools and then getting out of the way.
Ultimately this rule has no purpose or benefit. At best it is harmless; good DMs are just going to ignore it and give their players footnotes on what they can and cannot play in a homebrew campaign, and if issues arrive then hopefully they will come to a mutually fulfilling conclusion. Even in published campaigns DMs might still just ignore racial restrictions, such as if a player provides a compelling reason or the group comes to the realization that it is their game and they can do what they want. At worst it is hazardous for new groups, whose DMs might needlessly enforce it in a misguided attempt to "play by the rules".
I think the only thing that needs to be done is to add a footnote somewhere in the rules, that gives DMs that for some reason need it the "go-ahead" to ban/create content for their table.