I am on the fence about coming out. There is not a lot of discrimination against polyamory (though there have been some where parents have lost their children.)
I also see multiple marriages next on the horizon after gay marriage is legalized. So coming out now may help that cause of the future.
I heard once that people were more uncomfortable with polyamorists than with monogamous people cheating on their spouse.
The main purpose of coming out is so the general public can associate names and real relationships with a concept. I met many heterosexual people who changed their views on homosexuality after meeting a homosexual.
So far, I have told someone if they ask, but it is rare that such a question comes up.