Consider the following side line of the Advance Caro-Kann:
[FEN ""] 1.e4 c6 2.d4 d5 3.e5 c5 4.dxc5 e6 5.a3 Bxc5
In master OTB games, the move 3...c5 is one third as common as the main move ...Bf5. The main justification I have heard for this is that black is immediately conceding to a lost tempo by having first played first ...c6 and only then playing ...c5. I contend that this justification makes no sense, for two reasons:
(1) In the main line after 3...c5, black immediately regains the tempo when he recaptures with the bishop on c5. True, white can himself gain a tempo on the bishop with b4, but it is at least questionable whether this is the best move in the position.
(2) Even if we grant that black doesn't really regain the tempo because the bishop will have to move again, the fact remains that in the lines after ...Bf5, black still needs to play ...c5 at some point. But whenever he plays it, he will still be losing a tempo. Where is the harm losing this tempo on move three rather than on some later move?
So my question is: is the 'lost tempo' really the reason why 3...Bf5 is preferred to 3...c5?