For those who arent familiar...

i used to play the exchange a lot, but i found the line after 4.Bxc6 dxc6 5.0-0 Bg4 "nasty little pin" 6.h3 then h5 is very unpleasant. i am just going over the main line, which is probably best. it is playable but the Lopez is about giving black trouble and i find this line white has more trouble. now i know lots of players will disagree i am just say for me.

The resulting pawn structure is a plus for white (if a pawn ending were to arise, white can create a passed pawn on the kingside, but black can't create one on the queenside). However, the bishop pair is a plus for black.

The bishop pair is probably slightly more important, since the exchange variation scores on average slightly less for white than the 4.Ba4 lines.

It is not played as often as Ba4 because many players do not like the endgame, positional manuvering nor have the patience to exploit White's advantages.  The whole tenor of the opening has changed. Basically they see it as boring.  ML Ruy's  offer them more of what they want.

Although the DERL and DDERL (every play the CARL?) are probably better objectively than  E(a)RL they drop the main reason most club players use the ERL, limiting Black's choices.  Every additional move White makes gives Black the chance to foil White's carefully crafted opening scheme.

What line CA-Ruy Lopez? Center Attack?

The CARL (great name!)is the Center Attack Ruy Lopez.  There are 2 main variations centered loosely in Europe (5. d4) and the USA (6. d4), at least by the literature.

1. e4 d5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. d4

1. e4 d5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 6.  d4

The usual rap on the lines is a premature release of center tension resulting in less pressure on Black.  OTOH it is very playable and takes a lot of weapons out of Black's hand. 

When I play it I too use the 5. d4 variant.