At first I was going to say that the opportunity to back out and the need for more input are identical: if the dialog consists only of a button to proceed and a button to back out, the user needs to choose one of those as input, and eliminating that need for input means eliminating the opportunity to back out.

But now I'm thinking that a need and an opportunity are very distinct. For example, browsers used to present a Save dialog during a download: was there a need for input? No, accepting the default filename works, and based on that, they no longer offer the opportunity to choose a filename. Thus, "..." indicates the opportunity, even if there is no true need.