Wood is naturally strong, otherwise tree's wouldn't exist in nature in the first place
Wood itself is a good combination of rigidity and flexibility that allows most boats (assuming no rot) to be able to withstand waves and the odd bump from a trailer or rock at modest speeds. All those tall ships on the ocean is proof enough that wood is strong.
Wood gets weaker when it rots, so adding some kind of sealant like paint or epoxy gives it more resistance to rot, so it keeps its strength longer.
But paint and epoxy alone can be easily pieced or scratched off, exposing raw wood to moisture, which can cause rot and loss of strength.
Fiberglass cloth makes it a lot harder to scratch or otherwise expose the wood. That alone is worth while if you are loading & unloading your boat a lot, or operating near rocks or stumps or the like.
Epoxy will stiffen the wood a bit, but not dramatically more. Its not worth cutting down wood thickness to add fiberglass thickness. The rock that was going to punch a hole at 50 MPH in your 1/4" plywood will probabably still do so regardess of it it's glassed or not. If that is your operating conditions, look at aluminum or steel instead. Even a solid fiberglass boat will split under those kinds of forces.
When building your first boat, its easy to overthink this. Especially on things like transoms, bottom hulls, etc. No amount of material strength will replace good operating practices and common sense boat operation.
Fiberglass will protect the wood. Wood will take most anything but the freak hits, and glass will usually not change the results either way.
Now if glass made wood 3x or 5x stronger, then maybe. But the small percentage it adds to the overall composite (vs. uncoated) strength is probably significantly less than using aluminum instead.
The only place I don't suggest glassing is the places where you want to re-sand and re-finish from time to time, like a solid lumber top deck. Otherwise if it's plywood, and going to have ANY sort of wear & tear (kids, trailering, boat docks), then glass should protect your investment well past it's original cost in materials & labour.