The stated purpose from the site is to be able to match songs on a playlist, so like keep a consistent bpm across an entire playlist. If that was your goal, you'd presumably want the dotted-quarter bpm for 6/8 instead of the eighth. For a listener, the feel of the song is going to be based on when the kicks and snares come, not how things are divided up within those beats. That's what I was trying to get at, but man it's hard to put a description of time signature into plain text.