Why not take the bishop with the knight (Guseinov vs. Bauer)? In this game between Guseinov (2622) vs. Bauer (2634):

Crete 2007 · French, Winawer, Alekhine (Maroczy) gambit (C15) · ½-½

[FEN ""]

1.    e4    e6
2.    d4    d5
3.    Nc3    Bb4
4.    Nge2    Nc6
5.    a3    Ba5
6.    b4    Bb6
7.    Na4    dxe4


After dxe4, Guseinov played 8. Bb2.

Why did he not take the black dark square bishop on the b6 square with his knight on a4?
There is no hurry. After 8. Bb2 the bishop on b6 is not going anywhere. NxB continues to be available to white until black does something about it like a6. That means that delaying NxB gains a tempo if black has to make a less useful move to try and "save" the bishop like a6. Since recapturing with the a pawn probably gives black a better game it is worth waiting to see if he will throw away this option by playing a6.

Delaying the capture also retains the possibility of playing Nc5 which may be stronger in some circumstances.

Finally an immediate NxB will be met by axN opening the a file for black and allowing the a8 rook to pressure the now weak, backward a3 pawn.

So, in short, delaying the capture retains flexibility. That way white has more and possibly better options. This "maintaining the tension" is one of the things which separates weak and strong players. Weak players are in too much of a hurry to release the tension in a position. Strong players maintain the tension and with it give their opponents more opportunities to go wrong.