And why is that a bad analogy? The fmri results show activity at a resolution that would be equivalent to sayign SoCal has more road activity (and to further your analogy) and economic activity than say Wyoming.
Especially since most fMRI studies of note tend to be ones studying specific inputs. It makes total sense to see what parts of the brain light up with a given stimuli in a given set of conditions. Of course you wouldn't extrapolate that to the entire brain, as brain research is as much about what does light up as it is what doesn't.