I have a Linhof M679cs and a P65+, but I use the Canon T&S lenses more often. Here's the main reasons,
1. For interiors you're often really pushed for space, not in terms of "wide-angle" space, but in terms of "where's the photographer supposed to stand" space! A DSLR is just more compact and manouvrable, especially compared to a technical camera fitted with a sliding back.
2. Instead of complex interior lighting set-ups I'll often just use HDR, and managing the multiple layer files from a DSLR is quicker and easier than from a digital back.
3. The architectural photography I do is unlikely to grace the pages of Architectural Review, it's bread and butter commercial work for developers, hotels, and building materials suppliers. In this context there's not much reason to use the Linhof/P65+, the quality from a DSLR is more than adequate for four colour offset printing, and the movements available from the Canon T&S range are adequate for 99% of the challenges you'll face. That's not being cavalier with quality, it's just being realistic.
4. This may be heresy for many, but in my view the marriage of digital backs and technical cameras isn't that harmonious. There's certainly no cheap option, because the mechanical precision that digital backs require doesn't come cheap. But even with the best cameras you'll struggle to get sufficient focusing accuracy from the ground glasses currently available, and you certainly won't get that focusing accuracy with the viewing devices available for sliding backs. Consequently you often have to fit and remove the digital back for each shot and sooner or later you'll drop the back on a concrete floor. So better have your insurance up to date!
5. In the crowded European cities where I work you can bet that at least half the exteriors will need extensive retouching to remove parked cars, street signs, etc. In practise that means moving the camera to get a clean view and then patching in a section to cover up obstructions. It's a lot easier to move a DSLR than a technical camera, and it's quicker to complete the retouching on smaller DSLR files.