Sheathing with epoxy and cloth of some type has several functions. Depending on the boat and location, different functions may be more important.
1. Tensile strength. In this case, the fiberglass provides the tensile strength and the epoxy just holds it in place. This is important for impact resistance, and for holding together light strip-planked boats (which are really wood-core composites). George's point about epoxy-to-glass ratio applies here, particularly when weight is important. A special case is the seams of taped-seam boats, where the fillet is actually in compression sometimes, and the outer skin of cloth is mainly a tension member.
2. Abrasion resistance. This is important on decks, and on the bottoms of boats that are beached regularly or are used in white water. Other types of fabric (Dynel, Xynole, etc.) work better than fiberglass for abrasion resistance, although their tensile strength is lower. Again the fabric does most of the work, and the epoxy holds it in place.
3. Waterproofing. In this case, the epoxy is the functional element. The fiber reinforcement is necessary, however, to allow a thick enough layer of epoxy that doesn't crack and leak (think rebar in concrete).
4. Keeping fir plywood from checking. This is similar to waterproofing. The structural integrity of the sheathing layer, which needs to keep the plywood surface dry and immobile, is the most important thing.
"For a successful technology, reality must take precedence over public relations,
for nature cannot be fooled."