You are right that if no UV or eyeballs will hit the interior surfaces, varnish is only for the show-offs, in my opinion.  That said, I take John Harris at his word that the best defense of the wood, especially the okume, is a complete epoxy encapsulation.  I was trying for light coats on everything on my skerry, especially the non-fiberglassed surfaces, and had to go over things an extra time or two when postcure inspections showed some thin spots where I could tell from the change in sheen that I hadn't gotten the wood coated completely.  And this was before I started the final sanding.  Needless to say, there were some spot recoats when I worked through the epoxy with sandpaper by accident.  I gave up on the ROS sander for all but near flat/convex surfaces. Most of the interior was hand sanded.  The fillet interior corners were sanded w/ a contour sander.  That said, I managed a good painted working finish, but not a show finish by any means.  Patience and time ran out. But I did seal everything, and inside the bow and stern tanks, I slathered some extra resin because I figure if it gets wet in there, I'm less likely to spot it.

   I only used varnish on the seats. Everything else was painted. The manual states that paint is more durable.

   My rails and breasthooks are the only part of the hull that's varnished.  The rudder, tiller, daggerboard are varnished as well.  I'm pleased w/ how the varnish holds up.  It can get abraded, but it's easier to touch up varnish than the paint, IMO.  I epoxy coated and varnished the mast and spars as well.