Brian, this is another "hot" topic button!
I totally believe in a full gel! I think a soap is 99% less prone to get ash for one thing. Second, a fully geled soap is harder, firmer faster than an ungeled one. Third, I feel that during saponification, more heat is created and the saponification part of the process is more complete than in an ungeled soap. A more complete gel, a more complete saponification, me and a little less cure time. These are only my thoughts and observations. That is why every TOG Mold is sold with a tight-fitting wood lid. Like I said, my views only. Not belittling anyone that does not gel or even puts their soap in the fridge. I have always encouraged a full, complete gel in my soaps, and all I do is goat milk based soaps!
I was curious about this as well, so I did a search for the melting point of sodium oleate, to start (oleic acid is a common fatty acid). I came up with this interesting article about phase changes of sodium oleate at various temperatures. The gel phase appears to occur as the new soap reaches higher temperatures. If your soap goes through a gel phase, it means that the saponification reaction has generated enough heat for the new soap to go through a phase change. The saponification reaction is exothermic, which means that the reaction generates heat. You may not get a gel phase in smaller batches because the heat disappates quickly. In larger batches, or in those that are wrapped in a blanket after pouring, the heat will be contained and the soap temperature will rise. It seems that certain fragrance and/or essential oils also effect reaction rate and temperature.
I'm not very artistic, so I can't describe accurately the difference between soaps that gel and soaps that don't once they're all done. Maybe someone else could comment here?
Anyway, here's the link to the article; I hope this helps!
I really don't care one way or the other except that if it does gel, I get complete gel so I usually put my molds in the oven, with the oven warmed but turned off. I do try to not let my milk soaps gel as I don't want my mixture to overheat and separate and I love the look and feel of the soap when it has not gelled - it has a creamier look to it. Like Paul said, it really is a matter of opinion.