For me it's more a question of what you lose when you deny it. Every time
> you advocate something politically you are by definition saying it ought to
> be changed.
>
Not necessarily. "Ought" implies some external or objective imperative - if not God, then a convenient substitute. But one can also advocate a change simply because one wants it, or because it serves one's interests. (This is also what the overwhelming majority of those who go on about how things "ought" to be are doing, but in a manner that is either confused or dishonest.)
> Denying morality is essentially denying that there's any ought involved,
> which seems to me both bone-headed and needlessly arrogant.
>
Those who assert an "ought" should be prepared to define it. If others bases political positions on religious or philosophical systems that dictates certain things, so be it - we can consider their inspirations when deciding how seriously to take them. If they cannot begin to explain their thought processes, which is usually the case, the decision's really been made for us.
> It essentially asserts that your morality is objectively true. Besides
> being wrong, it's a terrible way to persuade persuadable people to join your
> side.
>
You seem to assert here that denying morality somehow asserts the objective truth of morality. I must have misread you, but perhaps you could reword your point here?
-- "Hige sceal þe heardra, heorte þe cenre, mod sceal þe mare, þe ure mægen lytlað."