Lots of people describe them as party cruises. I have rarely done them but I did a 3/4 B2B on Indy out of PC last July and another 4-nighter in October (just b/c of the price) and didn't find much of a difference from the 7-night cruises I normally take in the summer.
Maybe it's because in the summer there are lots of kids on any length cruise, maybe it's b/c I don't party or stay up super late, maybe just a difference in perception. Regardless, I'd have no problem doing a 3/4 B2B again (I plan to for the 1st two Utopia sailings unless the price is insane). It's the short length that I find unappealing, not the experience, hence the B2B's.