If I'm playing the Spanish as white, I don't take the knight, simply because my light-squared bishop is the stronger of my two bishops (since the black king starts and castles onto the light squares). If I'm playing as black, my dark-squared bishop is the stronger (because the white king starts and castles onto the dark squares). So if my opponent takes my knight with their stronger bishop, I see it as a mistake (even if the trade ends up with me having doubled pawns). It depends though, and my reasoning is by no means concrete! 
Spanish Opening Exchange
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Nothing wrong with the exchange. I like playing on the blackside of it, its very fun.
Grandmasters do use the exchange, it may not give white as much pressure but its still alright.
Jun 13, 2013
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#4
Players of a very low level indeed often take the knight thinking it wins the e5 pawn - not realising that Black gan regain the pawn with a good game. Once you start playing stronger players, you'll notice the Exchange Variation happening less - although as noted above, it's still a completely viable line in its own right as Fischer and others have shown.
Yes, I'm quite certain that it is a viable variation and a good player can do a lot with it. What I marvel at is that it is used almost exclusively. Even at my level, when I see the Sicilian, I see the dragon, I see the Najdorf... every variation from time to time. I just don't understand the overwhelming popularity of this one line in the Spanish. You would think it is against the law to vary from BxN.
I run into the Spanish a lot... no surprise but what I do find surprising is that they take the knight 100 percent of the time. I don't think I've seen anything else even once. When I look at grandmaster games, they almost never do. Why is that? Do so few players ever want to learn anything from grand masters? It isn't like there is some secret forced checkmate in it. I would go so far as to say I win most of my Spanish Opening games with black pieces. Even if there is some universal advantage in taking the knight why am I able to win most times and why haven't the grandmasters discovered its innate superiority?