Again, my main question is why take the knight (4. Bxc6)? Unless white just likes more open games it doesn't make any sense.

If you reach an endgame (take off all the pieces besides the pawns) this is supposedly a winning endgame for white, although it is very tricky. You have to play through the middlegame to get to the endgame though, so during the middlegame, you can improve on black's doubled pawn weakness. 

this is the mainline move. and lasker never said that, you have to move your a or h pawns. if you can't move them what use will they do? If your opponent moves a piece deep into your territory do you let them? no, you move your a pawn. kind of acting like a smart-ass a little hmm

for black this pawn structure is very solid, and has a useful open d file for the heavy pieces. black also has good dynamic chances, like with iqp strcutures.

It pretty much ruins the Ruy--all the tension & complications blown out just to get some miniscule (and alleged) endgame advantage, though it does have its fan club. Seeing it so often made me start playing the French.

chess is not an absolute game. rules can't go for everysingle aspect of chess. and ALSO, if you check the game explorer of many master and grandmaster games, you can see that a6 is the most common follow-up. take that to grandmasters who play the spanish and tell them that.

It's pure dreck, advocated mainly by forum trolls.

Bobby Fischer, who knew a little about chess, won several games with the exchange variation against top grandmasters

http://www.chessgames.com/perl/chessgame?gid=1044249

http://www.chessgames.com/perl/chessgame?gid=1044018

Parham loses a move with no compensation in the opening, and it goes against the principle of not involving your queen too early in the game.  You can use it because it may seem strong to you, but I'm not convinced with it.

Don't feed the troll. Thanks!