Bill,
it's interesting you say that - the build manual for my Shearwater doesn't call for it, and in looking at the (extremely well) documented builds that Etienne has posted on his website, he doesn't seem to do it either.

Does it really make that big a difference? Not questioning - just trying to understand.

Thanks,
Gero

I'm not Bill (obviously) but will add 'my two cents worth' here...

Bill may have been commenting on the option of glassing only the inside of the seam and just filling the outside with epoxy. Without glass on both sides, most of the strength of wood-core construction is lost.

Etienne's method of extending the deck glass across the seam certainly does work well for him (look at the results!).
For builders in areas with big humidity fluctuations, or folks who don't 'push through' with the build at a steady pace, having a deck sitting with the inside glassed and the outside bare wood is an invitation to problems.
So, IMO, for most of us, the 'standard method' of promptly glassing the hull and deck on both sides and later joining with glass on both inside and outside is a safer choice.