All the people who don't turn off their lights, think its cool to leave them on or think that it will make little or no difference is the kind of attitude that is holding us back.
It's a tricky argument, that one. I do believe it wont make that much of a difference though. You might say that's the attitude that's holding us back, but at the same time, how does turning them off move us forward. Even then, there's no possible way to get your cause into every single person. So, when there's a call for people to do things like this, they are only delaying the inevitable. That's why little projects like this have any kind of value or majesty to it. Less people do it, more people dont. That said, it's like ZL said earlier. It is a "symbolic" event. The largest problem is that different symbols mean different things to different people, just the majority believes that they dont care and keep their lights on anyway.
And, if you want to be a real hardass. "how" is that attitude holding us back. It is actually improving us in several ways. Even though it's slowly killing the planet, it's just a drawback that we have to decide for ourselves; a logical descision that we as a species cannot turn back from. Power consumption creates jobs, power usage has progressed our quality of life and the way we live our lives, and there is no possible way we're going to become independant of electricity. You say it's holding us back, but instead it's pulling
specifically the planet down. Also, bringing the planet down in the first place gives a whole host of other people jobs. Humans like jobs, if fucking the planet creates more jobs (even if the inventors didn't know themselves at first), then let there be jobs.