I'd say it is more a thing for parents, than for their kids. Parents worry that I kid won't like it or won't fit in. And some of them won't like it. But a lot of the events are one-offs, and if your kid sees something he/she likes, they are free to go to it and participate, even if they don't have a crew. Plus, my sense was definitely that is more of a case of one or two bad apples than a whole boat full of cliques.
Still, if your kid is on the shy side, going the first night could be helpful, as no one else knows anyone either. When I went off to college, I found that for the first 3 weeks, just about anyone would talk to me because no one knew anyone. So I made sure to talk to a LOT of people those first three weeks and as a result made a lot of friends with people I NEVER thought I would have gotten along with, but all I had to do was say Hi. Then, each year, a couple of buddies and I made sure to spend those first three weeks saying hello and talking to the Freshman. Consequently, to this day, I have good friends from each of the 4 years at that college. So if the cruise is like that in microcosm, figure that the first 2 days your children have a great chance to make new friends just by talking to people they don't know. It can be a great social opportunity for a child to practice being outgoing, knowing that they won't have to deal with any of these kids a week later if they don't get along with a certain person.
One other thing, my sons were Vibe age, but they did say that that first night was WAY more crowded (like 50 plus teens) than any other time on the trip, so if your child goes the first night and is intimidated by all the crowds, encourage them to try again the next day, as it will probably be much less crowded.