I'm not 100% sure. But IIRC the problem with normal USB storage is Android has to partition your phone for internal storage and USB storage. You computer can then umount the USB storage from phone and mount it in your computer. So in many phones without MTP, even though the internal storage had capacity like 16GB only 1 or 2 GB was available for app installation. While some phone gave upto 8GB for app, that space was wasted for people who didn't need that much for app but needed space for music and photos.
With MTP mode there isn't separate partition but a whole single partition. So if you have 16GB internal storage in your phone, you can use whole 16GB for apps, music and photos.
MTP mode is available from Honeycomb and I don't think its an optional component. I mean I don't think you can say I don't want MTP mode, I want USB storage mode.