Editing Issues

This comes from our [me and Rick's] super-quick editing session tonight, as well as my new fascination with the playwright Adam Rapp and his (in my estimation) genius-level plays Stone Cold Dead Serious and Faster, so bear with me.
One of the things I've been thinking about lately with respect to dialogue in theatre is what's important and what isn't. Obviously, I think poetry is always a huge part of what the theatre does. No matter what, poetic dialogue is not superfluous dialogue. It deserves a place in a play by simple virtue of its poetry. I feel much the same way about jokes, although with comedy there is always the question of timing. So much in the theatre is a spell that is easily broken and nothing is better at spoiling the magic than poorly timed comedy (okay, maybe bad acting). But jokes in theatre--I call anything that gets a laugh a joke, be it a sound cue or a hand movement or a punch line--earn their place simply by getting a laugh. They exist by virtue of their comedy.
The larger question has to do with dialogue that isn't comedic or poetic. My question is "what is the purpose of this dialogue?" Often the purpose of the dialogue is exposition. It's a little harsh to call exposition a necessary evil, but writers think about it this way a lot, and getting exposition to work well is very tricky. The other purpose of dialogue is character, and, by extension, relationship. Dialogue between two characters can create a relationship faster than any other device I can think of. It's faster than silence, faster than action, and way faster than a light cue or a sound cue. What I've been thinking about lately is that a lot of what creates a relationship onstage is not what's said but what doesn't need to be said. By that I don't mean things that are left unsaid, but things that are assumed between a couple, or even a group of people: shared history and ideals that creates the basis of a longstanding relationship but never needs to be mentioned onstage. The audience infers the facts they don't absolutely know, and the result can often be more interesting than if they were to tell their entire life-stories.
For me it works like the "magic secret" in acting. It creates a reality that exists for the actor, and even though the audience knows nothing about it, the secret fascinates them endlessly, because they know that they're missing something.