A wide angle lens is essential for shooting real estate, but an ultra wide angle lenses can be abused. When I first got my first real wide angle lens (a Canon 16-35mm on a Canon 1Ds) I loved to rack it out to 16mm and shoot a whole listing with it on 16mm. Then when I would show the results to my wife (the listing agent) she would go ballistic and send me back to shoot it over again because the property looked like a "bowling alley".
The fact is, as Simon illustrates in this tutorial, an ultra wide angle lens can give you some pretty wacky, exaggerated perspective that really looks strange to the eye and is very distracting to the viewer. Yes, there are many agents out there that want that "bigger than it really is" look, but be deliberate. Don't just blindly go wide because you can. There's a reason that most tilt-shift lenses are 24mm- it's because architecture generally looks more natural when shot around 24mm than 16mm, or 14mm.