To put it simply, the field of view of a camera body, which depends on its sensor size, determines the effective focal length of a lens when used on that body. There are a variety of sensor sizes and body depths, and therefor a variety of fields of view, for cameras these days. If we take just Canon, they have three sensor sizes for their DSLR cameras: Full-Frame 35mm (1x Crop), APS-H 28mm 1.3x Crop, and APS-C 22mm 1.6x Crop.
When it comes to lenses, a single lens may be able to be used on multiple camera bodies. Again, if we take Canon as an example, the bulk of their lenses are the EF mount. A single EF mount lens, say the 24-70mm focal length L-series lens, supports all three of Canon's DSLR sensor sizes (and therefor all three fields of view.) One may buy the 24-70mm lens for their first Rebel series 550D body, and later upgrade to a full-frame 5DMkII body. When buying an expensive lens that should have a very, very long life, the field of view of the camera body should not really be a factor.
The focal length of the lens itself is really the key factor, and as long as you know the appropriate multiplier for your sensor, you can calculate the effective focal length for each body it might be used on, and its usefulness on that body. This little fact was useful for one of my recent lens purchases. I have a Canon Rebel XSi (450D), and I needed something in the 24-70 range. Since I know my crop factor (or focal length multiplier) is 1.6x, it was easy enough to calculate that the 16-35mm L would effectively be a 25-56mm lens, which generally fit the bill. I also know that when I upgrade to a 5DMkII (or III) in the relatively near future, that this lens will behave as a very nice, ultra-wide to wide angle 16-35mm zoom lens ideal for landscape photography.
If lenses were rated in their field of view, it would be rather confusing to make such a simple determination as effective focal length when a lens is used on different bodies with differing sensor sizes. Lenses are lenses, and should be rated in focal length. Camera bodies are camera bodies, and there should be a simple way of determining their focal length multiplier due to the field of view the sensor provides. In most cases, cameras have a known multiplier, and if not, the information can be easily gleaned (Canon has 1x, 1.3x, and 1.6x, Nikon has 1x and 1.52x, etc.)