Remember grade school, with parts of speech, Conjunction Junction, and diagramming sentences? Well, think back to the last (non-trivial) sentence you actually said out loud. I challenge you to try to diagram that sentence.
Although there's quite a lot of research that's been done, surprisingly little of it deals with the way most of us really communicate.
That's not to say that current approaches to linguistics are correct; I don't know enough to make that judgement. But I think the problem lies more in the complexity of the domain - not because modern linguists are so foolish as to base theory on written language.
not because modern linguists are so foolish as to base theory on written language
That was actually what I was trying to get at. While there are clearly some rules we follow when speaking, they are much more open than those governing our writing.
It's odd, though, or at least counterintuitive, that our comprehensive of these less-structured spoken communications is higher than that of more-structured written ones.