I think the answer is yes and no.
I often leave behind a lot of sugars in the mash (which I sparge a little more to freeze for starters, but that's a different story), which I think is what fred is getting after.
for example, I generally collect 14gal for a 12gal batch, losing 1.5gal to evaporation over a 90min boil and .5gal to hop absorption and dead space to get 12 in the fermentor.
For an average 1.065 batch of beer, I generally acheive 77% efficiency in collecting my 14gal, but can collect about 3qts of 1.025 starter wort with another sparge while the boil is going on.
if I sparged another 1.5gal through the mash grain bed, I could get another 1.5gal of 1.025ish gravity beer for 15.5 gal but then I would need to boil longer to get down to my batch volume, in this case 3 hours total of boiling given my boiloff rate.
There would be a little more sugar in the wort at this point since I collected more sugars.
However, I think this is retarded since to save a few $2 in grain by being more efficient I wasted $5 in propane and polluted more as well.