I should feel guilty commenting on Plato, since I don’t read Greek, so strictly speaking can’t read a word he wrote.  I do feel that the translations of the early dialogues I’ve read suggest something less one-sided then was implied. Certainly, some other people writing in what they imagine to be that form seem to produce something that smacks much more of “monologue” and of “yes men”.  It seems to me that what Plato’s Socrates generally does is to get someone to make an initial statement (a definition, say).  He then gets the interlocutor to agree to a secondary statement that would seem to be entailed by that.  He then proceeds to show a contradiction between the two statements.  However questionable as a proceeding — for example, maybe the contradiction would no longer apply if one were to slightly modify the secondary statement — that’s an active and logical dialectical process.
Here’s one you missed: The _Compleat Angler_ is written in dialogue form.
I don’t know of any contemporary philosophers who have written serious works in that form.  However, Roger Scruton has written some satirical dialogues with Socrates’ supposedly shrewish wife Xanthippe as a disputant.  Here she’s no shrew, but rather a humane and witty woman capable of putting a callow Plato in his place.  Scruton is, I think, one of the more interesting and articulate contemporary philosophers in the English-speaking world, though controversial on account of his conservative (Burkean) political views.  Whatever one thinks of those, I have to say his _Xanthippic Dialogues_ are written with considerable verve and wit, and as a send-up of academic writing (as well as Plato) very funny — even the “index” (by H. P. de Selby — how’s that for a fake “Oxbridge” name?) is stuffed with in-jokes:
http://www.amazon.co.uk/Xanthippic-Dialogues-Philosophical-Roger-Scruton/dp/1856194027/