Well, a lot of nature books currently play Valshalla, especially because of her great bonuses against Iron Golems. Kralathor competes for that strategic role of "big creature that gets bigger". They're very different mechanically, but does a book want to run both? If not, is Kralathor better/more efficient?
It changes the nature book from one that wants to run lots of creatures to one that wants the opponent to run lots of creatures. This may depend largely on what opponents are playing.