In a nutshell . Yes
Lighter flies swim better, so by using sinking leaders you get the depth and present the fly better
Sandy
I'm not sure I agree with that Sandy, bit too much of a generalisation. A light fly will move more in the water but there are times like right now where you may not want the fly moving about too much. A lot of people use a weighted fly to get down on a floating line, this will give a slow moving fly fairly high in the column.
For me, the weight of the fly is about speed of swing and presence in the water more than it is about depth. In a cold water I want a heavy fly, in a big water I want a heavy fly, in these situations a light fly will often not cut it even on a fast tip as it swims too quick. Obviously if fishing in very low cold water then the fly would be lighter as a certain flow is required to make a fly of a certain weight swim.
Does that make any sense?
Cheers
Ben