user8291: After the moves
[FEN ""]
1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e5 6.Ndb5 d6 7.Bg5 a6 8.Na3 b5 9.Nd5 Be7 10.Bxf6 Bxf6 11.c3 O-O 12.Nc2

12...Bg5 is considered the main move, to preserve the Bishop pair and to activate Black's dark-squared Bishop. 
But White could easily take the same Bishop with the Knight at moves 11 and 12. Why is Black allowed to preserve and activate the Bishop? What are the refutations, at least as long as plans and ideas are concerned, of 11.Nxf6 and 12.Nxf6? I don't see ...d5 as an immediate threat from Black after Knight takes f6, so White could take the Bishop and perhaps follow-up with c3-Nc2-Ne3.

user3480: In the Sveshnikov variation, Black concedes positional advantages (weak pawn in d6, and outpost in d5) for dynamical play based on: active pieces, pawn break (...f5) and dark-square play. Indeed, White gives up their black square bishop early in the game meaning that they may lack of pieces controlling black squares.
It seems not appealing to exchange the strong d5 knight against the bad bishop in f6, therefore White usually develops a classical strategy in this case: put all your pawns on the colour of the missing bishop, i.e. White will try to put pawns on black square to control some black squares and then compensate their missing bishop.
In the position you point out Black has played two other alternative moves in practice: 12...Rb8 and 12...Bb7 and against both the strongest for White is 13.h4! (scoring very well in practice).
Why White does not play 12.h4 ? Well, White need to take care of two goals: firm control of the d5 square (otherwise they have no advantage) and fight for the dark square.
In case of 12.h4, Black has time to fight for the d5 square :
12.h4 Be6! 13.Nc2 Bxd5 14.Qxd5 Ne7 15.Qd3 d5! (and the d5 square and the d6 pawn are no longer a weakness).
Something similar may happen in case of 12.Nxf6, then for instance this could follow:
12.Nxf6 Qxf6 13.Nc2 Bb7 14.Ne3 Ne7 and 15...Rd8, 16...d5 cannot be prevented.

user291: Black would be thrilled if White exchanged his perfectly-placed knight on d5 for Black's bad bishop on f6. 12...Bg5 brings it to a diagonal where it's actually doing something and also makes it possible to eventually play ...f5, undermining the pawn supporting White's knight.
White doesn't play Nxf6 earlier because he'd be exchanging a well-placed piece for a badly-placed one. Note that if Black plays alternative moves on move 12 like 12...Bb7 or 12...Rb8, White still doesn't play 13.Nxf6.

8291: <p>After the moves</p>

<pre><code>[FEN ""]
1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e5 6.Ndb5 d6 7.Bg5 a6 8.Na3 b5 9.Nd5 Be7 10.Bxf6 Bxf6 11.c3 O-O 12.Nc2
</code></pre>

<p>12...Bg5 is considered the main move, to preserve the Bishop pair and to activate Black's dark-squared Bishop. </p>

<p>But White could easily take the same Bishop with the Knight at moves 11 and 12. Why is Black allowed to preserve and activate the Bishop? What are the refutations, at least as long as plans and ideas are concerned, of 11.Nxf6 and 12.Nxf6? I don't see ...d5 as an immediate threat from Black after Knight takes f6, so White could take the Bishop and perhaps follow-up with c3-Nc2-Ne3.</p>
 3480: <p>In the Sveshnikov variation, Black concedes positional advantages (weak pawn in d6, and outpost in d5) for dynamical play based on: active pieces, pawn break (...f5) and dark-square play. Indeed, White gives up their black square bishop early in the game meaning that they may lack of pieces controlling black squares.</p>

<p>It seems not appealing to exchange the strong d5 knight against the bad bishop in f6, therefore White usually develops a classical strategy in this case: put all your pawns on the colour of the missing bishop, i.e. White will try to put pawns on black square to control some black squares and then compensate their missing bishop.</p>

<p>In the position you point out Black has played two other alternative moves in practice: 12...Rb8 and 12...Bb7 and against both the strongest for White is 13.h4! (scoring very well in practice).</p>

<p>Why White does not play 12.h4 ? Well, White need to take care of two goals: firm control of the d5 square (otherwise they have no advantage) and fight for the dark square.
In case of 12.h4, Black has time to fight for the d5 square :
12.h4 Be6! 13.Nc2 Bxd5 14.Qxd5 Ne7 15.Qd3 d5! (and the d5 square and the d6 pawn are no longer a weakness).</p>

<p>Something similar may happen in case of 12.Nxf6, then for instance this could follow:
12.Nxf6 Qxf6 13.Nc2 Bb7 14.Ne3 Ne7 and 15...Rd8, 16...d5 cannot be prevented.</p>
 291: <p>Black would be thrilled if White exchanged his perfectly-placed knight on d5 for Black's bad bishop on f6. 12...Bg5 brings it to a diagonal where it's actually doing something and also makes it possible to eventually play ...f5, undermining the pawn supporting White's knight.</p>

<p>White doesn't play Nxf6 earlier because he'd be exchanging a well-placed piece for a badly-placed one. Note that if Black plays alternative moves on move 12 like 12...Bb7 or 12...Rb8, White still doesn't play 13.Nxf6.</p>
