Not sure where you heard that people are not doing gowns. We did the formal dinners on our Mariner cruise in a Tux(me) and a gown (wife). My wife packed her gown in a garment bag (put it in a dry cleaner plastic bag to prevent wrinkles) and had no problem.
If you want to dress up, go for it. If you don't want to go through that much fuss, then you could also do the cocktail dress. Its up to you.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013