I hate that people always go "I love them because if you get them with Petra it's super powerful". Well guess what, Petra is super powerful for anybody. Petra + normal trade post (or mine on hills) is also super powerful!
I think the Kasbah is decidedly underwhelming. They come very late, even later than Trade Post, which means you will actually put off developing those tiles for a while (or otherwise first put trading post and then replace it with Kasbah) which is bad unless you have other and better tiles to work. Kasbah + hills makes for a decent tile, but so does a normal desert mine. I completely agree with steveg7000 that Kasbah should increase to +2 Gold with Economics, that would make them much more viable, and I think they should become available earlier.
As a UI, I think this one comes out a little short, it simply does not offer that much of a benefit. Sadly, the same can be said about the Maroccan UA.