We usually call for a layer of fiberglass cloth on the outside of the larger boats. This is not so much for structural reasons as for weather resistance. A glassed plywood boat can last a very long time. Any of the boats can be glassed or not, it's up to you (we suspect most people glass the larger boats, and not too many the smaller ones).
We use a layer of 4oz. to 6oz. cloth and resin on our boats. Many people prefer to build our boats using epoxy resins (and glues); we used to use other resins and glues, but for the past decade or two, we've switched to epoxies. Try to find and use epoxies which are in a 2:1 ratio — it's easier to measuer and mix these. Epoxy is now our First Choice for glassing wood, and glueing wood joints like the stringers and any other parts (which other resins can not do). Epoxy also gives far longer working time than polyester or vinylester resins. We have used polyester and vinylester in the past, and still do, for certain eperimental projects. Vinyester is used like polyester, but has some better properties. As noted at the beginning of this question, the glass and resin (of whichever type you chose) adds weather resistance and some strength (and weight), but can be left off if you're trying to cut costs to a minimum. Most people chose to glass their boats though.
Fiberglassing is not as hard as it might sound if you know a few tricks: dry fit the cloth so that you can get the major wrinkles out before you start catalysing anything! Use a paint roller (not a foam roller, as we found out the hard way!) to get the resin on in a hurry. You want to move fast, so slop it on with a roller and then get fancy with a brush to get the details. A squeegee can be used to get rid of excess resin (helps find air bubbles too), but don't try to get the cloth too dry. The wood will soak up a bit of resin before it cures. If you get the fresh resin and cloth all nice and dry, you'll come back in an hour and wonder why your cloth is too dry. We're not going for maximum strength here, just a layer of glass to help protect the boat. The Weekender video shows how to fiberglass if you think you want to see an example first.
Overall, epoxy is the way to go!