About 12...Bg5 and move orders in the 9.Nd5 Sveshnikov After the moves

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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.
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.