This, too, has a logic to it: while some kinds of punctuation are traditionally reserved for joining full clauses (periods) and others for dependent clauses (commas), ellipses and dashes are deemed acceptable for joining both sorts, even in the most conservative styles. So if you’re writing informally and you don’t want to bother deciding whether your string of words is a full sentence or merely a clausal fragment, one way to split the difference is to punctuate ambiguously—to use an ellipsis or dash.
