There may have been a misunderstanding (Maybe it's me?)... My car (2010 GMC Terrain) has a USB port that I can plug devices into (Flash drives, phones, iPods, iPhones, etc) and it allows the car to read MP3 files off of these devices and allows you to control file seeking through the car stereo's controls. No bluetooth support for music either; phone calls via bluetooth only. So in other words, it isn't the phone reading the car, but the car reading the phone.
With my current phone (ATT Galaxy S2) my car will not read it at all if it's in MTP mode. I had to install a custom ROM that supports UMS. UMS makes the car see the external memory card as a flash drive.
Every single custom ROM I can find for the LG Optimus G Pro does not support UMS (stock doesn't either). They also use MTP. It's just that I always use my phone as a phone AND an MP3 player both on the go and in my car. I use it to play music through headphones, but also in my car. I'm not really willing to give up car functionality and cannot afford to buy a separate MP3 player for the car.
Unless USB Host somehow makes the phone compatible with my car stereo, I don't think it is of any use to me... right?
EDIT: According to my car's manual, the USB port can only play PlaysForSure (PFD) devices, Zunes, iPods, and USB Mass Storage Devices.