On both of my Celebrity and HAL cruises the women in actual evening gowns was maybe 20%. Now they did dress nicer on those lines-you sae no one wearing jeans and shorts on casual nights but still they did not dress as formally as I was lead to believe.
Many women simply had on black dress slacks with sparkly tops. On the informal nights I saw little differanc eindress then the formal nights.
I can't speak to Holland America Lines, but I can tell you that I have never seen more than a few women wearing black pants and spangly tops on any of my cruises with Celebrity on the formal evenings, and I'm an "Elite" member of the Captain's Club. That outfit would be fine on "informal" evening, though. In my experience on Celebrity's formal evenings, nearly all of the ladies seem to dress in keeping with the guidelines -- either a long dress or a formal cocktail dress.