I think the main use of the softpipe driver is as a "reference" platform so that (a) upper level functionality can be built and tested without having to wait for hardware drivers, and (b) if there is a question about the behaviour of a specific hw driver it can be compared to the softpipe reference rather than having to find different hardware for A/B testing.
From a user point of view, however, unless there is a good hw driver for your card then you shouldn't be planning to use Gallium3D, although the llvmpipe driver might be turn out to be interesting if you have a big honkin' CPU and weak graphics.
Softpipe is the safety net. Not every GPU is going to come in at once. As long as theres a soft pipe they can finish without waiting for a billion ducks to get in a row. Some of those cards are probably being 4 and 5 times more work than the "average" gpu.