I think that's to do with how Android works. Unlike iPhones I don't think you can play music over USB, but you can mount the phone as a hard disk, which the head unit can then access, but I think that requires the music files to be in certain places on the phone's SD card. However, that's a small flaw that I would happily live with to have a much better phone! The same goes for Windows Phones, but they're even better than Android!

I just use Bluetooth streaming- Cortana on Windows Phone will happily play stuff by voice command and is very accurate, I'm sure that Google Now can do similar things.
Although I've a feeling someone posted a picture of there being a some way of being able to unlock Mirrorlink with a code from the dealers?