In a previous comment someone mentioned small client budgets as a reason that they don't use supplementary lighting. I am trying to say that no budget is too small (at least anything that is even worth considering shooting for) that someone can't bring a few small flashes or inexpensive halogen lights for at least occasional use.
Thats true. If you have the time, you can add some supplemental lighting for very little money and inconvenience. I do it sometimes. But when the budget is low and time is very limited in a space or building and the client wants many shots then generally available light and a DSLR with shift lenses is king.
Regarding Camera / Lenses.
I would love to use medium format, mainly for the sometimes superior optics, color and dynamic range but I find that I use the 14-17mm focal length range quite a bit on interior spaces and I just don't know if there are some viable alternatives for those lenses in a MF Digital body or a technical camera with an MFDB. Phase One / Mamiya has the 28mm which on the largest MFDB sensors is about an 18mm equivalent on 35mm full frame and Hasselblad has the new 24mm which is about a 17mm equivalent. I have seen samples of the Phase / Mamiya 28 and its ok but not great and I have not found much info on the Hasselblad 24mm. But, the Canon 17 and the 24 have shift and are really good lenses. I love to use shift on a lot of the images I make to maximize the use of the sensor.
Now the D800e is also in the mix. not much because of its resolution but forits amazing low iso dynamic range which is a huge asset in architecture photography if you wish to deal with high contrast scenes with single image capture.
Also, there is the unique Hartblei HCAM. I do not have much info on this camera but looks like an amazing option.