There is no real reason why other formats could not be supported in this plugin. It just requires putting an up/down list of formats in the configuration dialog (allowing you to set your own preference order for which format to get the count from for when there are multiple) and a couple of ther tweaks. The code is already in place to behind the scenes convert the book into a format that allows the count to be performed - it just means that the plugin will run slower when it hits nasty things like a book for which you only have a PDF format.
However as it would allow users to have better coverage of page/word count in their libraries without having to convert to EPUB/MOBI just to get one it sounds a nice addition. I'll put it on the list for the next version (no promises on when it will get done at this point).