How would it have looked to have someone not in uniform at these negotiations?
Downright insulting, I'd think. Which is why I seriously wonder why Jellico insisted on the uniform...
Really, he should have asked for Guinan to join in, wearing her most outrageous hat while bringing drinks and chitchatting with the CO on what to do with these lizard men and their silly demands. The bartender could have launched into a story about her uncle, too.