I get it, Denny. I have repitched the entire contents minus the break and dead cells, which is pretty easy to accomplish in a 5-gallon-size batch by leaving behind about a pint of green beer, swirling the culture into suspension, and waiting a few minutes for the break and dead cells to sediment out before decanting the cells that are still in suspension. That works for a few repitches, but ultimately culture degrades over time. I serially repitched a true top-cropping yeast strain that I acquired from the NCYC in England a dozen times without a degradation in performance. That is when the practice of discarding the brown head and cropping the second head became crystal clear. This particular culture was not the easiest to use due to being a Yorkshire culture and Yorkshire cultures are notorious for having high O2 requirements. If a brewer is not careful, Yorkshire cultures can come out of suspension early and leave a brewer with a diacetyl bomb.
Here is the brown head forming from that culture:
Here is the second head that formed after the brown head was skimmed:
I could repitch the second head indefinitely without a degradation in performance. Sure, the culture would adopt a house character from selective pressure, but it would not become powdery or start to develop off-tastes due to an ever increasing dead cell count. That is what happens when a brewer who serially repitches does not select from the middle of the cone. In the end, does a brewer have to select the cells from the middle of the cone? Well, not if he/she only plans to repitch the culture a handful of times before returning to the reference culture. However, for those who want to repitch a culture until it adapts to their brewery, selecting from the middle of the cake is not an option, nor is plating the middle of the cake after a number of repitches. The goal is to select cells that perform well in one's brewery.