I can see the opposition to including hunter-gatherer cultures and the like, but some of them deserve less scrutiny than others. On one end of the scale, we've got, let's say, the Inuit. While it would be great to have a civ that can work snow (and I've seen some pretty cool Civ V mods showing how they'd do that), I can see why people would be opposed to a civ with little impact on world culture and no established cities... although a lack of major cities didn't stop the Scythians...
As for Australian Aboriginal people, I'm kind of for, kind of against. It'd be hard to pull a UU/UI out of it, so maybe just include an Aboriginal leader as an alternate for John Curtin? I can see Windradyne taking that place. (Also, I have no problem with Australia being in Civ the same way I don't have a problem with America being in Civ.)
Setting aside the need for there to be a leader and significant historical record of the civ (to pull uniques and abilities from), I think it comes down to a strong internal culture vs a strong impact on world culture. Take for instance, the Cree. Rich history and culture to draw inspiration from? Most certainly. Overall impact on the history of the world? No offense, but, not much. Most likely, they were included because 1) Everyone wants at least one Native North American civ (sorry, Aztec, you kind of straddle north/south), 2) They'd never been included in Civilization before, and 3) The Cree were Civ's answer to Canada, as the Aztec are their answer to Mexico.
Heck, one could make an argument that the ancient Indo-European (or Proto-IE) people(s) had a major impact on the world's culture, language, and history. We've been able to find out a surprising amount about their culture by studying their artifacts and burial sites, and, while a UU/UI would be hard to devise, we already know of at least one man who, given the rare (at the time) copper axe he was found with, could very likely have been a chieftain. His true name is lost to history, but the modern world knows him as Ötzi the Iceman. They'd be the blob civ to end all blob civs, but who's to say the Indo-Europeans are no less deserving, simply because they don't have as great a record as Egypt or Sumer, or even the Cree?
(Postscript: Wow, I wasn't even angry, yet that turned into more of a rant than I expected...)