"Need", no, although it's certainly preferable. Comics, as a mixed media using pictures and words, is a balancing act so both aspects have to be considered. As others have said, a comic with amazing art but a poor story will draw people in but not retain them (depending on how poor the story is, anyways). Likewise some people may never look at the comic with the best story in the world if the art is particularly unappealing to a lot of people.
That said, I do think that the story portion holds a little more weight. As long as the art is at least passable, a good story can easily carry a comic imo. On the flip side if the art is nice but the story not so good, I'd rather just follow the person as an artist rather than a comic author, kind of xD;;
edit: the situations in quesiton though are assuming extremely good/bad art/writing. The's definitely a wide spectrum of skill levels in between and the truth is most people falls in the middle somewhere. I don't feel as though someone has to be really really good at either aspect to make a successful comic :] and both skills improve over time/with practice.