This may have been just our experience but when we took our first cruise in 2006 on the Magic, all 6 of us had complete pirate costumes. It was the majority of people that were dressed up. We had a riot!
Flashforward 2 years later on the Wonder in 2008 on a 4-day cruise. There were literally only 2 families in pirate costumes at our dining time. We honestly thought that we got the evenings mixed up. You would not believe the looks we got from people on the ship like we were a family of freaks! We were laughing so hard because we were so embarassed. My daughter was 15 at the time and mortified. Our servers of couse loved it though.
We have 4-day cruise on the Dream in Feb and the big joke with the kids is "Do you want to dress up for Pirate night?" and the response is always a resounding "NO WAY!"