The Spiro ad agency in Philadelphia in the '80s (it's long gone now) had a slogan that I thought was the best ever for an ad agency.
"Where good enough isn't."
I miss those days in the advertising industry, when copywriters and art directors worked together to create brilliant slogans and campaigns, to sell the sizzle and not just the steak. Or the hamburger, as the case may be. Today, the online revolution has meant that the bean counters have taken over and (rightfully so, I must admit) made it all about the numbers. These days, more often than not, good enough IS good enough. However, if you're the one who's charged with bringing together a marketing communications program - the world is so much bigger than just advertising these days - then you'll probably face the decision at some point as to whether you should hire an art director or a designer to develop and produce your online and print vehicles.
If you need an annual report produced (one that's already been written), or a newsletter template or other piece for which the "marketing" is pre-determined, you may do well with the services of a designer. Because they don't need to focus on the bigger marketing picture, they can concentrate on creating a beautiful design that's easy on the eyes and directs your audience(s) attention to what's important, no matter what the medium.
If you're looking for a creative idea, however, and someone with experience at corralling lots of different specialties to develop campaigns, it probably makes more sense to consider an art director. The better ones combine project management skills with exceptionally good taste. The best ones tend to have worked at an ad agency and know what it takes to sell stuff.
And isn't that the reason you're marketing in the first place?