It should only turn Bluetooth on if you're transferring a larger file (say an image or video file). NFC, while low power has a VERY slow transfer speed (about 53 KB/s at maximum). For example, if you wanted to transfer a 8 MB image, it would take around 2.5 minutes. Now it might not seem like a long time, but keep in mind you have to have both devices touching (or near touching) for that entire period.
Instead what Google has done is that when Beam detects that you want to send over a picture/video it will automagically turn on bluetooth, pair the devices, send the data and turn off bluetooth. Pretty convenient in my opinion.
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