What are the benefits of glassing a kayak inside and out?

The glass on the outside gives abrasion resistance and strength, interior glass adds significant strength.  A puncture is much less likely to happen if the inside is glassed.

Under point loads, such as hitting a large rock, survivability is much better when the inside is glassed. Also, a layer of glass does not weigh that much more than enough fill coats to properly seal the wood.