So, when you are using a yeast cake, you are really just supposed to use a piece of the cake? So, am I an idiot for using the entire cake?
Confusing terminology.
When people say "pitching on a yeast cake," they mean they whole thing. Plenty of people do it and undoubtedly make great beer.
The fact of the matter is that the recommended pitching rate for ales is 0.75 million cells per milliliter per degree plato (double that for lagers). This is what the experts (Wyeast, etc.) say. If you call that the "correct" pitching rate then pitching onto an entire cake, in most cases, is over pitching. Just as pitching one tube or smack pack is under pitching. IMO, this is not the debated part.
The debate lies, however, in whether over pitching makes a noticeable difference. Yeast make unique flavors during the growth phase, if you pitch essentially enough yeast where no growth occurs you will not get those flavors at that point. Now the question comes up: "Does that matter?" I think this is the point people debate. It is entirely possible that the flavors that a yeast makes during the rest of fermentation make detection of the earlier flavors impossible or that other parts of the beer cover them up (hops, roast, etc.).
Someone really needs to do some side-by-side experiments. Ferment out a batch, harvest all the yeast, use the Mr.Malty calculator (or better yet a microscope) to determine the "correct" amount of yeast to pitch. Pitch the "correct" amount in a subsequent batch and double (or triple) that in another. Ferment the beers out noting lag and and time to FG. Then, most importantly, do some triangle tests and see if people can detect the difference and if either is preferred. I would suspect that people would be able to detect a difference and have a strong preference only in certain styles (not hoppy ones, etc.).
Why not? There's plenty to recommend it- it's a guaranteed, fast-starting fermentation that hits FG quickly.
I would (and many others) argue that if you pitch the proper amount of healthy yeast (0.75mil/ml/degPalto) you will get a fast-starting fermentation that hits FG quickly. (This has been my experience). More yeast is simply not necessary for this to happen.
Have you read any of Jamil's recipes? Some of them call for starters > 2L...that's a LOT of yeast. I think I remember seeing one that was 7L. And most commercial breweries aim to have fermentation over in 3-4 days MAX. Fast fermentation means less chance for infection, lower power bill, and quicker to market.
But all of those starter sizes are based on 0.75million yeast/ml/degPlato (double that for lagers) in a simple starter from one pack/tube. Some big beers do require a lot of yeast/big starters. I think Jamil would even agree that making starters this big is way too much work and pitching from a previous batch is easier. I would also argue that commercial breweries that re-pitch are not over pitching, they harvest the yeast and split it up or pitch into a larger batch. I have never heard of brewery repitching the entire yeast load from one batch to another.