Yup, vital for our family of 8! I got what the TA called one of the last on our cruise, and we booked 11 months out. I sure hope
DCL puts some sort of priority on those for people who need them, instead of offering them to people who only need one cabin.
The lowest category for them is Cat 10 (just like cat 9 but inside only). If you can, get a secret porthole room. I know 2 pair of those rooms connect, but not sure which ones. You get an obstructed window view for about $200 cheaper than to get the cat 9. They're on the 5th floor near the lifeboats.
Connecting rooms go all the way up to Cat 3 suites connected to a Cat 4 family deluxe room, or even two Cat 3 suites. Some people will say these are the "smaller" cat 3 suites, but hey, a suite is a suite!!
With connecting rooms you can have 8 people total (cat 10 to cat 5), or 9 people (cat 4 and cat 5/6, not sure which one), or 10 people (2 cat 4's or a cat 4 and cat 3 or two cat 3's).