A B2B has too many repeats. I'd rather do a more exciting itinerary with different shows and menus each night.
That's what's holding me back from doing a B2B, too. Also, I think the turn-over of people on the ship would feel odd. My kids would have to make new friends again.
We've done 2 B2B and would do them again in a heartbeat. We worried before the first one (2 5n on Wonder) about both of these concerns - we love seeing the shows and eating in the MDRs and our DD is a little shy, so worried she would have a problem making friends twice in the clubs. Neither was an issue for us.
The repetition of the shows and the menus was actually a benefit to us - there wasn't as much pressure to see all the shows on both cruises, so we ended up skipping them entirely on the second cruise on our first B2B, and spacing them out a lot better on our second B2B. We relaxed more, enjoyed the deck, the lounges, DD hung out in the clubs, and we could still be sure we saw the shows we enjoyed. As for the menus - another added bonus! We usually end up ordering the same favorite menu items, but with a B2B, since we got each menu twice, we had more opportunities to explore other things we wouldn't normally try (because we have to have our favorites!) We also had 2 cruises worth of Palo experiences, which is always a plus.
As for the club experience for a shy kid - not a problem. She really didn't have issues with that - especially since we made sure to find some other families on our cruise on the meet thread and get them in touch before-hand. That worked well, and she had a blast.
I think the biggest benefit to a B2B is on the turnover day - instead of being sad to be getting off the ship and going home, it's more a bit of excitement that you're starting another vacation! It's sad to see the friends you made from your meet thread go, but there's a whole other group embarking with you for the next adventure as well.