Here's my take, for what it's worth, as a Texan.
I may support open carry as a concept, but look what happens when you put it into effect. A flood of complaints to restaurants, who then close their doors to OC, legal or not. A flood of newspaper stories, showing people openly carrying long guns. My understanding is that, in some cases, the restaurants are simply asking people to leave their long guns at home (since OC'ing handguns is illegal everywhere in Texas). Why? In response to customers who don't really want to be eating next to a guy with an AR slung over his shoulder. I don't blame them.
I would never call for a ban on that kind of behavior, but I'd ask those who insist on OC simply because it's not illegal to reconsider. There are hordes of people to whom firearms are way down on their list of issues -- until the aforementioned AR owner sits next to them and their young children. Then, it's likely they will take the simple step of emailing the restaurant chain and saying they're done patronizing that chain until they refuse to allow OC on the premises. They also will likely look negatively at future attempts to improve gun laws here and will make their feelings known to those who they elect to represent them in Austin.
So you've turned people who were on the fence about guns or even leaning towards support in the other direction. We lose a potential supporter and maybe all his or her friends. Is it worth it?
Rather than seeing this enhancing the cause of OC, I see it as a major roadblock. OC of handguns will likely come up again when the legislature reconvenes next year, but with all the negative press, I'm guessing it will once again fail to garner enough support to pass.
We in Texas are blessed with a very supportive legislature and governor, and with a concealed carry law that is effective and has attracted hundreds of thousands of gun owners to carrying concealed, to the benefit of all. Sabotaging that good will by opening carrying a long gun in public is counter-productive and threatens to cost us all the progress we've made. As many others have said, just because you CAN do something legally doesn't mean you should.
The original NRA statement was a bit strong, and I wish they has just stayed out of it, because it will cost them some of the membership gains they've made since the left has made gun control one of its key issues.
If you're one of the OC folks who have been staging these rallies, please reconsider. You are not doing your cause, or the cause of the legal carry of firearms, any good.