I don't see why they would need to do this. There's literally no incentive for Google to add support considering not only would they not receive any money for it, you can easily install another reader app to read whatever format you want. This isn't like a Kindle Fire or Nook Tablet. On those platforms, I can understand including the ability to read non-native formats since there's no normal way to install another reader app.
I completely disagree.
I will start using Google Books if and
ONLY if I can upload my existing books into it just like you can do it with Google Music.
I simply refuse to have 2 separate "libraries". If Google will allow me to upload my existing books, I will start using Google Books exclusively and will absolutely be buying books from Google only.
I wouldn't be using Google Music right now if they didn't allow me to upload 100s of songs. Since then every single song I've purchased was from Google instead of Amazon.
So yes, there's a $$$ incentive for Google to add this relatively speaking trivial feature. Unless there is some authors royalty issue, all Google has to do is use the Google Music architecture and tweak it for Google Books.
Until then, I don't see Google Book going far.