Life is full of compromises. We use Canon because the compromises we had to make were ones we found acceptable. Nikon would have required other compromises, which we found unacceptable for us. For someone else, the reverse may be true.
For example, I think the Foveon sensor is the way to go, but if we had gone with the Sigma SD10, we would have been locked into Sigma-only glass. This was not an acceptable compromise.
With our 20D and 30D, we can use Canon glass, or Sigma, or Tamron, or Tokina. In fact, one of my wife's favorite lenses is a Sigma (the 150mm macro). By getting the Canon bodies, fewer compromises were made.
To say, however, that Canon is the end-all and be-all is purest BS. Each brand has its own advantages and its own disadvantages. The hardware purists lose sight of the fact that the object is to take pictures, not to own gear. One should use whatever equipment serves that end best for that individual.