1D MKIV is a sports camera. Having a 1.3x crop factor gives it better reach (but unfortunately no EF-S lenses will work on it).
I think once you get above the XXD the cameras start being less about individual features like the Rebels are and more catered towards specific uses. The XXD is going to be well rounded, a clear step up from the Rebels but not really geared towards one thing. Then the 7D is clearly a birding/sports camera....There's no reason to get it over the XXD unless you need the speed and AF system (which I really don't). 5D is the compact pro camera for landscapes and studio work, but you really aren't going to take it over a 7D if all you do is BIF. It doesn't have a very good AF system and it only shoots at 3.7 fps or something like that. 1D MKIV is most definitely a pro sports camera as evidenced by watching any televised sporting event. All those white lenses and vert grip bodies = 1D MKIVs and L tele primes. then the 1Ds MKIII is kind of in an odd spot right now since it's old...It's supposed to be the end all be all (short of medium format) landscape camera, but most of it's feature set is out performed by most everything else in the line (still has a single Digic III processor) and it's image quality on the sensor level is matched by the 5D MKII. It still has one of the best AF systems in any of their cameras, but otherwise it's not really a good choice. It's supposedly next up for replacement though, along with the rumored 7D MKII.