How many times have you leaned over to clean blood off of your hands or a knife? How many times have you done that when the deck is slippery? Or in a decent sized swell? Sometimes everything comes together at once, slippery deck, sudden swell, leaning over the side... and you're in the water.
I made it back in with no troubles, but it was enough to make me glad I had my inflatable vest on. Even though I never blew the vest, it provided a convenient place for my buddy to grab hold and pull me back in.
Wear your vest.