It does take away from the ambience! Would you go to a wedding or a funeral in a tanktop and flip-flops? Why should respecting a request for a certain amount of deportment be any different?
Frankly, if people can't be respectful of the dresscode then perhaps they should choose another line or vacation venue. There are plenty of other options out there, including Club Med and Windjammer, where you can wear whatever you like...go to dinner in jeans or shorts and tee shirts or whatever.
I respect that people are on their vacation and want to be comfortable, but there are other people to consider besides themselves. Flame away, but this is how I feel and my mind won't be changed.