Bishop takes knight, why?

Oh yeah. I bet she loves that.

Ah, I completely missed this one.

Quite true. In Pump Up Your Rating, Axel Smith offers the following game to illustrate that:

In some positions, a knight is strong enough that it's worth trading a rook for it, never mind a bishop.

Piece values are all about what the different pieces are doing, and capable of doing, in a given position. The pawn = 1, knight = 3 etc is just what people starting chess learn to get going. It bears little relevance in a lot of positions.

Another important theme of all this shows up in the 17th move of the Fischer game chessmicky shared: The side that does NOT have the bishop pair will aim to trade same-color bishops in order to neutralize the previous decision to give away her own bishop pair. 

One thing I learned from Maurice Ashley's commentary on Caruana vs Nakamura at this year's Sinquefield Cup was related to trading a Bishop for a Knight.  One of those things that sounds so obvious once you hear it, but which I'd just never thought of.  I'm used to thinking along the lines of "dark square weaknesses" when someone gives up their dark-squared Bishop, but I hadn't thought of the obvious flip side to that:

The person with the two bishops and one knight has only two minor pieces to defend a given colour complex.  If you have traded your dark-squared bishop for their knight then to counter your dark square weaknesses you still have 3 minor pieces that can attack your opponent's light square weak points.

That isn't something I've ever (at least not knowingly!) really put to much use though as it isn't quite as easy for someone of my level to take advantage of as the more obvious tactical weakness of a bishop missing from a colour complex ith pawns not compensating for it.