Steve,

I don't have muuch experience building boats yet, but there are some basic structural reasons for glassing the inside of the cockpit, especially on the bottom. I believe that the fiberglass/epoxy layer is very strong, when put in tension. This adds strenght to the kayak, by stiffening the overall glass/wood/glass sandwich. One could draw a comparison to the properties of plywood.

I used 3mm plywood on my Merganser. I have not actually paddled it yet, but I have observed the increased stiffness of the plywood panels after adding glass on both sides. The inside bottom of the cockpit has a continuous layer of 4 oz glass, with only one coat of epoxy. All other inside seams received 6oz tape. The outside bottom panels were reinforced with two layers of 4oz glass, while the deck only received one layer. I have applied the last coat of epoxy on both sides and Including a cockpit coaming, which I layed up of about 10-12 layers of 4 and 6oz glass, the kayak weighs 24 pounds (that is if my scale is correct).
I should mention, that I am not planning on surfing in rocky terrain. I don't think the layup is quite sturdy enough for abuse like that.

The point of my very long winded response is, that the extra layer of glass does not add an overwhelming amount of weight, especially if you don't apply filler coats as Julie already pointed out.

Good luck,

Angela