For me, the reason is because it's a pain to change the battery and it's a pain to keep the extra batter charged. After having owned the Razr Maxx HD, you really learn to appreciate high capacity batteries. My stock battery gets me through the day, but it doesn't let me do the unexpected (go online to research something for an hour straight - in addition to the normal workload)
Also, like owners of all electric cars, I get "range anxiety" I don't like seeing that yellow battery icon!