One of the advantages of rooting the Kindle is that they could be purchased for cheaper than most tablets of comparable ability. I believe that the equation changed with the newly announced Nexus 7, but that is very recent.
I think that also the prevalence of rooting activity on the forums has something to do with the fact that the type of person who goes out to a forum to learn about their device tends to also be the type of person that likes getting "under the hood" so to speak.
I rooted one of my earlier Android devices, but I found that the differences in the various versions of Android were not really relevant to how I used my device, so I joined the "leave it stock" school of thought.
There are some who dislike the Kindle Fire interface, and for them, side-loading another launcher while leaving the underlying OS alone would work, but I think there is a tendency to go all the way if you change anything.
I would personally like to see more forum activity about the stock Kindle Fire, but I think with the user community on Android related forums, that is unlikely to happen. Also, there may not be that much to say about a stock device that everyone doesn't already know.
But, good question...