Only reason I know of why people put a strip of tape over the fiberglass skin is if they expect extra wear along the edges, and want that thicker tape to serve as protection from nicks and scrapes.

The fillet (on the inside of the boat) however is a structural element. The bigger it is, the stronger the joint. Adding a layer of tape makes that fillet effectively larger and stronger. If you built a S&G boat and dissolved away the plywood you would be left with something that looks remarkably like a frame for a skin-on-frame kayak. The fiberglass fillets are effectively cast-in-place fiberglass chines, replacing the wood chines on traditional SOF kayaks. The plywood of the S&G then is a thicker, more durable replacement for the skin fabric. Because of its strength, and the strength of any deckbeams, you can eliminate internal frames connecting the chines. Using a continous layer of glass fabric to line the inside of the kayak serves the same purpose as using tape.

Of course, using a single layer of 2-ounce fabric will not be as strong as using two layers of a 9-ounce tape, so the theory needs to be adjusted to suit real life conditions. Generally one layer of tape, or a 4-ounce fabric should do. You want to avoid weak fillets as much as you want to avoid overkill. Too much looks ugly and wastes materials and your time.

PGJ