I think when walking, standing, or dancing a Jacket is elegant, but sitting down it looses it's elegance. So when going to dinner where 95% of time is sitting if you want leave it in cabin.
However, Some people can be elegant without a jacket, for some a jacket is a must to make them slightly less inelegant.
The NHS amongst others says that rather than BMI the better measure of healthiness (for men) is that your waist is less than half your height , so a 6ft man should have a 34" waist. The same rule applies to elegance, if you pass the waist test, you can look elegant without a jacket. If your XXL shirt is straining at the seams , then a jacket is a must. I know not a fashionable (PC) statement to make.