Accessibility is almost certainly a large part of the reason that the greeks tended to write in dialogues. The greek dialogues tend to be very approachable, due to the way they have to step through issues.
Don't get me wrong. I like Plato (quite a bit). But dialogues seem as approachable to me as poetry. They might be approachable in the sense that, since they simulate a conversation between two people who might not know eachother, they necessarily lead to a much clearer exposition of whatever is the topic at hand. It's indeed as if you were reading two people try to make their ideas across when they're complete strangers eachother. But it takes completely different kind of "reading" from the one a person might use when reading prose or when reading a pure exposition.
Two cents.