I think that being out would be useful if it involves simply treating poly relationships like mono ties. By that, I mean that one isn't trying to foist off on unsuspecting mono folk the fact that you're poly and have eight partners, but that one simply mentions relationship obligations in the same manner mono folk mention having to do something for/with a husband or wife.
I'm all for letting people know I'm poly in that fashion. It's part of my life and I mention it the same as I mention I'm married. It provides an air of normality to the whole equation and lets newly exposed mono folk digest it on their terms.
I doubt I'd ever be interested in a poly equivalent to Act Up or parades or the like. The parades and such cloak everything in carnival dress, to where the monos can take the approach of "let's go see the freaks!" I don't think that's a good way to garner a reputation for being reasonable people who engage in relationships just a bit differently. If one acts like a freak, then one should expect to be treated like a freak. Conversely, if one acts like a reasonable person, then one can expect to be treated like a reasonable person.
So, media events that are simply informative I think are good. Any media event that presents poly folk in a carnival atmosphere I think would be bad. And expecting most poly folk to line up to announce to their communities involves a carnival atmosphere--do mono married folk all line up to broadcast far and wide the fact that they're married?
Finding examplars of the poly community willing to share their stories is good. Offering up a examples of professionals, white collar workers, and blue collar workers, triads, vees, small networks, and large networks to highlight the diversity of poly people and arrangements is good. Moving beyond using only examples and expecting all poly folk to put on horse and pony shows is not.
I also fear that such a day would involve far too many attention whores in the community who relish trying to shock "mundanes" showing up looking and acting as freakish as possible, which would only work to associate polyamory with groups on the fringes of society. That's diametrically opposed to gaining acceptance as something that perfectly normal neighbors might be involved in. The greater the spectacle organizers try to create, the greater the danger of the spectacle turning against their aims.