[ChrisV]

I'm not sure, but I think this relates to the mouthfeel of the champagne. It's about how much carbon dioxide is crammed into the wine. Champagne with a "fine bead" is more likely to have a "soft mousse". Bead is the term used to describe the appearance of the bubbles, while mousse describes how the bubbles feel in the mouth. A "hard mousse" is an undesirable attribute.

There might be other reasons why a fine bead is believed to be better. I will say that usually wine nuts are correct about what makes a wine better or worse even if the idea doesn't make immediate intuitive sense. For instance, wine newbies tend to scoff at the idea that glass shape makes a big difference in the way a wine tastes, but I can assure you that it does.