Many of the sparge brewers I talk to about BIAB scratch their heads when they hear that the brew in a bag method is a no sparge method. Nearly all of them say "but what about all the sugars that get left behind in the grain, aren't you losing valuable fermetables?"
At issue is not whether no sparge "loses" fermentable sugar, but how much is actually in the boil kettle. While it is true that the grain bag after lifting from the mash tun has sugars remaining, at what point is sparging a zero return effort?
The concept of brew in a bag is maximum efficiency with minimal equipment, which correlates to less time and expense. To sparge a brew in a bag is counterintuitive - I say, "what's the point Wilbur?" When you lift the bag after mashing, the wort is 100% pure - that means if the water to grain calculations are accurate and the pre-boil gravity is on target, adding more water (by sparging) dilutes the wort to less than 100%. Sparging at this point reduces the gravity in the kettle and to hit the target gravity, increases the boil time.
One might say that the increase in sugars in the kettle through sparging is "worth it", and through boiling the concentration of sugars is greater. In theory that sounds valid, but the dilution of the wort counters that argument. Effectively, when the bag is lifted and a gravity reading is taken, adding wort with a lower concentration of sugar will lower the gravity in the kettle and increase volume - beyond pre-boil volume calculations.
Just last week while "assisting" a fellow brewer we tested this theory by measuring the pre-boil gravity in the kettle at 1.061. After squeezing the bag, he feels the need to "sparge" with two quarts of water because it makes him feel better; which means there is no applied science reasoning for the action. By the way, he has a full three tier two burner Blichman rig. So, he lifted the bag, squeezed, and then dumped the two quarts into the open bag which was hanging above the kettle. We caught a sample of the sparge run off mid-way through and found a gravity of 1.054 - seven points lower. Adding two quarts of lower gravity wort reduced the overall gravity to 1.059 and increased the volume - which he didn't need.
Sparge a brew in a bag? "What's the point Wilbur?"