I think you got that wrong. The guy initially tried to have the bike read data from a USB port and had to go to a MP3 hardwired into the 3.5mm port. There is no reason to use an MP3 AND a USB stick on the Spyder as you stated. The Spyder is in a sense dumb and essentially just a set of speakers, it needs music played into it.
Check again, in Lamonts video, his BT adapter was powered by the USB, but sound flowed via the 3.5mm jack.
If for no other reason, plugging the MP3 into the USB will save the rider from needing to recharge his MP3.
Regarding your 2014, for you to BT anything into the bike itself, you must have some type of BT receiver to go from the Tablet into the bikes GPS audio input. Possibly you run a Sena Dongle or similar.
In regards to the update from the early style round DIN connector, to USB, granted these ports could offer charging when needed, but essentially allowed the owner to utilize a standard USB to Apple cable to run an Apple device from the trunk. Items such as MP3 still required the 3.5mm audio connect, and were only controlling volume, whereas an Apple device was fully controlled via the Audio control on the left handgrip area. Similar to the 14 and prior, just did away with a round DIN special cable.