1) Two bathrooms is worth a LOT to us. Even the split bath on
DCL is not nearly as useful as having two full baths for a family of 4. Did I mention that I have a teenage daughter?
2) In our experience, sharing a cabin with 4 gets claustrophobic. We like to nap occasionally or let one or two people sleep in or go to bed early. Keeping flexible schedules is much more feasible when you have separate spaces.
3) My son and I are both light sleepers. With four of us in one room, anytime someone (DH) gets up to go to the bathroom, it wakes up either me or my son. Then, that person tosses and turns until the other one wakes up too. Then, we both get to listen to DH snore the rest of the night.
Two cabins usually costs more than sharing one, but never anywhere near twice as much. It varies by sailing, but usually 30-50% more than sharing a cabin. On other cruise lines, we have several times paid LESS for two cabins than sharing one!
For us, having more space makes for a more relaxing, enjoyable experience overall.