I think I'll take a stab at my own question since it, in hindsight, appears to be a poor one.
17...Rac8 develops a piece and avoids any potential loss of tempo with, say 17...Bxc5 18.Bc3 Be7. Also, the c5 pawn can't be saved. Finally 17...Rac8 invites b2-b4 when a7-a5 is at least slightly better for Black.
In essence, it might be a "developing, save a tempo, wait to see what the opponent is going to play" type of move.
I've looked around for the same positions in other Caro Kann sources. 13.Kb1 is not a common move with the position in question. Neither is it critical according to Neil MacDonald, p.88 of Main Line Caro Kann. He likes 13.Qe2 but says Black is still fine after that move.
The position after Black's 12th and the moves that lead up to it seem to be very important though, according to MacDonald.
Well's does not cover this line with near the same degree of thoroughness that Houska does.