Marvell calls it a dialogue, but is it? In the genre sense, yes, because the poem involves two characters who are chewing the fat about some philosophical issues.
But if you read closely, you'll see that with all this talk there's actually very little conversation. In other words, the body and soul aren't actually talking to each other; instead, they're addressing some rhetorical "who." In fact, since neither of them bothers to address any of the problems brought up by the other, it's like they aren't even listening to each other at all. It's only in the final stanza, when the body whips out a "thou" at line 32, that this "dialogue" starts to sound like one.
So why call it a "dialogue" at all? By using that term, Marvell's title sets us up for two viewpoints. And, truthfully, that's what we get. Instead of emphasizing the conversational give-and-take of the body and soul, though, this "dialogue" refocuses our attention on the issues: what's being said, who's saying it, and whether it makes sense. In the end, the dialogue is really a tri-logue (if you follow us), since both the soul and the body are making their claims much more for the reader's benefit, than for each other's. In that way, we are the ones who get the final say-so on who has it worse: soul, or body?