First, I don't think it's seful to address the issue in terms of rhetoric or rhetorical effects, which takes us away from the question of structure. An utterance can be rhetorical or not whether it's a phrase, sentences, grammatical or not.
This utterance has the subject-oriented emotive quality of an "exclamation"(as opposed to a other types of utterances -- question, declaration, command, etc.) and the missing verb or verb phrase is not so much missing as "hidden" or assumed, which makes this an elliptical expression, a structural characteristic that is typical of exclamations. This type of ellipsis speaks to the wonderful compactness of English. Compare: "What a nut case!" (This person is such a nut case), "How nice of you to help me!" (You are very nice to help me"). These two examples are not traditional "sentences" because both are missing a main verb marked for present or past tense; yet from a pragmatical standpoint, they are grammatical, as true utterances. In the utterance, "And what better symbol...than...", the speaker exclaims/marvels in an ellipsis that is expandable as "What can be a better symbol than..." As a final thought, I think beyond discourse concerning grade school literacy, we shouldn't be hung up with the traditional definition of a sentence.