I'm a big fan of the seal coat, but there are many people on both sides of that fence and really either one has good points to make. My points are as follows:

1) Less chance of having what you witnessed happen

2) You can sand the precoat and get out any last little blemishes that didn't show up before. For instance, a lot of times wood glue will look clear until you put the epoxy on and it prevents the epoxy from absorbing into the wood and it turns out a big light spot. It happens to the best of us. With pre-coating you can go back, sand, and fix it before it's buried under a layer of 'glass. It may not be a surface coating of glue. It may be where glue was and you scraped it off, but what you didn't see was what had absorbed into the wood which ultimately keeps epoxy from soaking in.

3) It makes the wet-out a little easier

4) When you sand the precoat you're also filling any final little gaps which will make for a smooth finished product with less chance of bubbles under the glass. Case in point: I found a few gaps that were very tiny when I precoated that if I had not found them would probably have trapped air under the cloth and I would have had to poke a hole and inject epoxy. Since I precoated I was able to fix it then and there.