As a marketing agency, your work is designed to reach people and be publicly viewable anyway, right? So I don't see the problem. Why not share specimens that have already been shared with the world at large?

Today's competitor may be tomorrow's coworker or collaborator. Perhaps your peer might wish to hire you or bring you on as a partner some day ... or vice versa. Maybe the overlap between your services and skill sets is only partial. In some cases, I and my competitors team up to work with a single client; so we actually refer leads to one another.

If you're aloof and standoffish, then you may be closing doors to future opportunities. Since the risk of being open appears minimal, just relax and talk about the best work you do.