Bravil, Leyawiin, the Imperial City, and Chorrol all have lower class dwellings. Skingrad is an upper-class town, Bruma dwellings are built downwards to make them habitable, therefore giving extra room in any house, Anvil (at least the area outside the docks) is a city that prides itself on beauty, and Kvatch was destroyed. As for Cheydinhal, I got nothing.
Chorrol's got
three lower class buildings. Imperial City has the "Waterfron District", a completely failed attempt at lower class area. The rest of both cities were middle and upper class, where (well, almost) everyone got at least two floors, and a basemant to add to that.
As for Bruma: What about those who can't afford to dig into the ground, then? They wouldn't just leave, they build their shacks somewhere too. Because that's what lower class people do, obviously creating a "lower class district".
And it doesn't matter how beauty-fixated Anvil would be, there would still be poor people, who cannot afford those fancy stone-houses the rest of them. And the docks does not count as a lower class area, as nobody actually lives there, exept for those who has their buisness there.
The same goes for Skingrad. There is no thing as an "upper class town". Towns are dependant on the lower class. Especially Skingrad, even as it was portrayed in game, which relies heavily on lower class workers to keep it's graqe growin' and wine production going. Skingrad did have lower class models though, and even though they were extremely unrealistic in all their grandiour, they fit in quite well and I liked them. Mostly because of the "narrow" alleyways. Still, there were far too few lower class buildings and I would have liked it even more if there were some of the "shack" variant too.
Oh, and as Bravil was supposed to be the "http://tvtropes.org/pmwiki/pmwiki.php/Main/WretchedHive" of Cyrodiil, it isn't very strange that it featured lower class buildings.
Cyrodiil is located in the middle of the provinces and is a "melting pot" of cultures. Besides that, Bruma's dwellings, aside from the nordic carvings in the wood, are built for the purpose of conserving heat.
This is still no explanations as to why you can't find, for example, "Chorrol style" buildings outside that city. Or Skingrad architecture anywhere else than in Skingrad. Or why Cheydinhal doesn't have buildings in any style other than the one influenced by Morrowind culture. Or any of these "schools" in the IC.
So...it isn't a rip-off of something.
Seriously, very few people are arguing that Oblivion ripped off http://en.wikipedia.org/wiki/Europe, they're saying it's http://tvtropes.org/pmwiki/pmwiki.php/Main/MedievalEuropeanFantasy.