Yes. Washington state ferry system is a bit nuts. I have been left high and dry for 5+ hours because they were having issues with a ferry, meaning the route was under capacity, and the backlog meant 100% of seats on every ferry off the island that day went to people with reservations. I had to get out and yell at them in order for them to open up space on the next ferry for some of the cars without a reservation to get on.

Basically, if you don't actually need to get where you're trying to go, or for that matter, need to get back, and are happy to spend potentially up to a day or longer sitting in a ferry line watching other people board boats, feel free to skip the reservation.