I have AppleCare for my series 3. Although nothing major happened so far with the watch, I used it for long hours at a time almost daily. The watch is especially useful as a fitness tracker and have accumulated my fair share of scratches on the glass surface.
So yeah, it really depends on your usage. Do you do a lot of outdoor activities with the watch like rock climbing, swimming, kayaking or outdoor camping? Then maybe you should consider getting one.
I am upgrading to a series 5 and I have placed an order for the space grey aluminium. That has AppleCare too.