Just got back from a 3-night on Mariner. Night 2 was "Dress Your Best" night and one of the reasons the hubby and I took my mom and my sister on their first cruise--they love getting dressed up in really nice dresses. Personally, I love getting dressed up too; my husband is not so keen, but does it for me.
All this to say, if it's your thing, great. If not, no worries as the "dress code" is more of a "dress suggestion."