LittleToe:
It's right at the bottom alongside burning an American flag, in my list of things that genuinely make a point and a difference in life. "Yayy, look at me, I can burn an American flag, I'm not scared of your guns, tanks, grenades and nukes, etc.!" "Yayy, look at me, I can cuss at GOd on video. I'm not scared of your lightning bolts and hell, etc."
Certainly, denying the existence of a mythological entity seems like so much tilting at windmills. But to a lot of people, the holy spirit is a real thing, even a person. Many of these people are afraid that if they even question the existence of this being, they will be condemned, without possibility of reprieve, to an eternity of the worst sort of torture imaginable. What the Blasphemy Challenge does is show that there are a lot of people who are not afraid of this threat, who are confident enough in their disbelief that they will speak out about it. This may encourage others to question what they have been told and even break free from a lifetime of indoctrination.
I'm not even sure it has theraputic value to those engaging in it, though it does take a cheap potshot at the beliefs of a significant section of humanity.
Those beliefs deserve to have potshots taken at them. How dare anybody tell a vulnerable and trusting child that if they even think the wrong thing, they will suffer in agony forever! How dare they mentally rape a child with such a vile and ludicrous threat! Those beliefs deserve no respect or deference, and they will get none from me.