Personally, there is only ONE reason I would bi-amp: to bi-amp the BIG WOOFERS of a BIG speaker to actively control the volume of the bass.
But I think if the question is 1) bi-amp using an external amp vs. 2) bi-amp using the same AVR, then I think we can all agree that using the external amp is better.
Does bi-amp using the SAME AVR makes sense? I mean most AVRs will go from 120WPC x 2Ch down to 50WPC x 5Ch anyway.