knight and bishop combination

During a lesson on exchanges in Chess Mentor I encountered a position in which the author wanted to teach me that in that position a bishop & knight duo was to be prefered over a bishop pair. The funny thing is he seemed to expect me(the student) to be able to understand that just by looking at the position.

I have never heard of any situation where it would be preferable to have a bishop and a knight like that. To me it has always been "knights in closed games and bishops in open ones". Can anyone please enlighten me as to when one would rather go with one of each?

Thanks! 

The combinations of minor pieces that work best in a particular position depends on how each piece is most efficiently deployed in the position. Bishops like open diagonals and don't like their own pawns on their own color. Knights like the security of advanced outposts (outpost = protected by a pawn, can't be attacked by a pawn) 

I'm going to go out on a limb here. In the diagram you show, there is only one outpost for white. So, a second knight would be redundant. In many cases you find your pawn structure makes one bishop better than the other. In the diagram posted, the light squared bishop would be less valuable than the outposted knight, so it is worth trading it to maintain the knight.

In considering what minor pieces you should keep and what minor pieces to trade, you should consider how effectively each piece is being used. Some minor pieces are worth more than others! Figure out if knights can take advantage of outposts and if bishops can take advantage of open diagonals.

Sometimes a position with one outpost still enjoys the benefit of a second knight, either to protect the first from capture on the outpost or in a defensive manner. Sometimes one knight can take advantage of multiple outposts. There are no absolute rules in chess, just guidelines for strategy.