When to Knight Pin

I notice in many opening variations it is popular to pin a knight thats on c3 or f3 (c6 or f6 accordingly) with a bishop either against the queen or king.  I also know the bishop pair is considered valuable.

  My question is when is this acceptable and when is this a wasted idea?

It's certainly useful if you're trying to attack (or defend) a pawn or piece in the centre, and by pinning his knights you prevent their profitable movement.

I used pin knights this way as a matter of course, but these days I do it less as a FM recommended I play my bishops to c4 and g4 instead.

Most often I use it to create doubled pawns as HotFlow stated.

If done early, it can prevent the opponent from castling on the side with the doubled pawns.

Cheers!

Thanks folks