Taking the knight on c6 in open Sicilians Why is it generally uncommon (unsound?) for white to capture the knight on c6 in open Sicilians?

For example (this is just an example; I'm not referring to this specific position, but similar positions in the opening from Dragons, Najdorfs, Sheveningens, Richter-Rauzers, etc.): 

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1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 Nc6 6. Nxc6


I understand that not capturing the knight gives black the semi open c-file. But at the same time, capturing closes that c-file and gives it an isolated a-pawn and a semi-open b-file. Is it just because giving away the b-file to black is more dangerous than giving the c-file? I also understand that capturing the knight gives black an extra pawn in the center. But shouldn't white be able to quickly castle on the king side and torment black's a-pawn?
White's d4 Knight is his best piece. Why would he trade it for a common Knight and improve Black's center?

Regarding harassing the a pawn... you can have it.  While you're doing that, your opponent will be drawing a bead on your King.

Checking 365chess.com, we see that Nxc6 is a blunder at the sub-master level.  Black wins twice as often.

At the master level, Nxc6 is not played.  There are 2 games, the latest being from 2004.  Both ended with wins for Black.

The first game is from 9/4/2001 at Glifada.  It's an ECO B56.  Andreas Kofidis is White. Marijan Petrov is Black.  White is completely destroyed in this short game after Black sacrifices a Knight on f3.  Very nasty...

Edit - ...but survivable.  White lost instantly on 17. Bxf8. It's hard to believe he didn't see the mating attack since it's pretty easy.  So he must have though he was already lost.  Instead, 17. Qe3 holds.



[fen ""]

1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 d6 6. Nxc6 bxc6 7. Bd3 g6 8.  b3 Bg7 9. Bb2 Qa5 10. O-O Ng4 11. Qe1 O-O 12. h3 Ne5 13. Na4 Nf3+ 14. gxf3 Qg5+  15. Kh1 Qh4 16. Bxg7 Bxh3 17. Bxf8 Bg4+ 0-1




This game is from 3/7/2004.  It's ECO B58.  Albert Bokros is White.  Veniamin Sergeev is Black.  Sergeev puts his Queen out there and Bokros tries to trap it.  It doesn't work out, and by move 22 Sergeev dominates the center.  White seems to be trying a cheap shot mate-in-one on move 33. Perhaps he's under time pressure.  It doesn't work out and he drops a piece.

Edit - I see now that the Knight on a5 is trapped. Black piles on the pieces until it falls.



[fen ""]

1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 d6 6. Nxc6 bxc6 7. Be2 e5 8.  O-O Be7 9. f4 exf4 10. Bxf4 Qb6+ 11. Kh1 Qxb2 12. Qd2 Qb4 13. Rab1 Qc5 14. Na4  Qa3 15. Nb2 O-O 16. Nc4 Qc5 17. Be3 Nxe4 18. Qe1 Qd5 19. Bd3 Qe6 20. Na5 d5 21.  c4 f5 22. Bc2 Ba6 23. Bb3 dxc4 24. Bc2 Kh8 25. Bd4 Bf6 26. Bxf6 Rxf6 27. Bxe4  Qxe4 28. Qb4 Rg8 29. Rbe1 Qd5 30. Rf3 h6 31. Qc3 f4 32. Rf2 Rf5 33. Re7 Qxa5  34. Qh3 Re5 35. Rxe5 Qxe5 36. Qf3 Qe3 0-1





But what's important in both of these games is that Black is granted counter-play out the wazoo - a Sicilian player's dream.  We see Queen sorties in both games on the a5-d8 diagonal.  The b6 square allows the Black Queen to get a bead on the traditionally weak (yet dangerous!) b2 square and also peer menacingly at the castled enemy King.