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: The glass or phenolic glass spheres make the end result lighter weight (which
: means you don't have to use near as much epoxy), and easier to sand.
: That's the only reason they are used at all.

I agree with the rest of your analysis Doug, but there is another reason for using microballoons or other aggregates in this case. Quantities of epoxy as small as 6 oz. in a cup can generate exothermic temperatures exceeding 350* F. Besides the risk of setting the container on fire, the epoxy itself will expand, crack and shrink. Any aggregate will reduce the ratio of epoxy to total mass of the mix, thereby modulating the exotherm. Solid aggregates absorb more heat than hollow ones like microspheres, but of course they are heavier.

The most compelling reason for the endpour is to waterproof the seam at the ends where you can't reach. The method described in Nick's book and restated above wherein a large bag is placed around the end of the boat, which is then placed in a bucket of water, seems to do a fine job of preventing runaway exotherm from wrecking your boat, at a cost of nearly nothing.