I would like to know why it is so disadvantageous to have an advanced artificially isolated pawn when it can be exchanged by another pawn. I refer to situations where a side cannot mantain such square with a pawn.
Examples: In the Caro-Kann, after 1.e4 c6 2. e5 d6 3.exd6?! What is so wrong with this last move? Is it a matter of tempos (White lost two moves to go to e5 only to be exchanged ) or is there any more fundamental reason? I know that the common thing to do for White is trying moving pawns to d4 (maybe f4, though it d be weakening).
Another example, for instance is when Black c pawn is on c5 (with a White pawn on d4) and then it is pushed to c4 before recommended. Then if White can play b3, Black tries to play b5, a6. Why not simply trade this pawn already?