Personally I don't look forward to lugging the extra baggage that carries my dressier clothes and my husbands tux or dark suite...BUT...I do believe that the formal nights are a nice touch. They can add to the cruise experience.
Most likely the tradition harks back to a time when people did dress up more for special events...even for Sunday dinner. (are you old enough to remember those days?) Men wore hats and women dresses, but we live in a more casual society now.
Cruising and sailing in general includes many many traditions. Some can be fun...a glimps into the past, an opportunity to share in something that has been done for generations.
Some traditions are a way of connecting the past to the present. Of course not everyone feels this way...some may find it more of an inconvenience, and God forbid that anyone be inconvenience!
When we cruised last with Celebrity...not everyone on formal nights wore Tuxs or formal dresses...but all were dressed very nice! Some wore dark suites or a sports jacket with nice slakes...some of the women were in evening dress, others in nice silky pants suits, etc. NONE were in jeans, shorts, or super casual casual summer dresses.
It may take a while for past traditions to catch up with present customs...Enjoy them while you can...You never know, you might later have a change of heart and miss them.