For the NSA or FBI you mean? Perhaps if they have the storage space for trillions of encrypted messages for billions of users. I'm thinking that servers and storage limitations will see a shelf life put on them, and that a warrant would be required to access them if it's even possible to do so. I also think that Google wants carriers on board so that Verizon ect can use their own servers and storage space as it's probably not economical for Google to build the facilities required to handle every single Android user.
They're not doing for targeted ads or in real time, that much we know. From Googles policy;
" With end-to-end encryption, no one, including Google and third parties, can read eligible messages as they travel between your phone and the phone you message. "
Personally, I'd be more concerned about what happens to the encrypted message or file once it's decrypted on the receiving device. People get all caught up in the transit of the message without giving a thought to what happens to it on the other end. In other words, Google may not be able to see your dlck pic's but the person you sent it to, or the person who hacked your recipient can still choose to put it online for all to see. ;-)