"What's the harm in a thick mash?"
No harm whatsoever. The reason that a thick mash works with the decoction method has to do with preservation of enzymes. Basically, a thick mash preserves enzymes, that will be needed later on during the various temperature rests. At the end of a properly performed decoction mash and according to the style of beer being brewed, two to three qt/lb can end up as the final volume of water used throughout the process. It just isn't dumped in all at one time. The important thing to remember is that mash viscosity changes throughout the process and the mash may need to be thinned. Because mash viscosity can become very high, fly sparging becomes real tough.
I dough in with one qt/lb of cold water to allow mash pH to stabilize, caused by the inherent pH of the malt and the pH of the brewing water. If mash pH is acceptable, I will remove the first decoction and step it through a proteolysis rest and an alpha conversion rest, before bringing it to a boil. While the first decoction is resting, I fire the mash tun, raising the main mash up to 95 to 100F and let it rest at that temp until the end of the decoction boil.
Mash will jell after it is boiled. When that happens, an entire different circumstance begins to take place, enzymatically. During that time and depending on the rest period, beta or alpha or both at the same time, will reduce the jell (amylopectin) to A and B Limit Dextrin. It is Limit Dextrin that is responsible for the body and mouthfeel in beer, not necessarily from non-fermentable sugar formed during mash temps in the alpha I and II range.
