It probably depends on where you are. Thus the following will be US-specific: extrapolate with care.
No. It may have some practical utility for the participants, but fighting AGW isn't it.
N.B. it's not that climate change is wrong or unimportant. Protesting just isn't necessarily a good way to combat it. Better options: writing your congressperson, writing your representatives in your state legislature, signed petitions, campaigning for politicians likely to take concrete steps. All of these raise attention in a way that doesn't turn the concerned parties into a salable packaged product for the media.
The main reason protest is unlikely to be effective is because most of the "solutions" that fit on a piece of posterboard are going to be comically unworkable (e.g. "stop greenhouse gas emission now!"). More viable steps to help the problem are going to be relatively complicated and boring and long term (e.g. carbon credits, along with taxing trade to countries that pollute heavily). The pie-in-the-sky nature of these soundbite demands invites the opposition to dismiss one as clueless or ideologically-influenced or what-have-you.