Ulterior motives aside, if this saves even a handful of kids from growing up with porn addiction, its probably worth it, right?
If you think that all mistakes with the blocker will be ironed out, then you don't have any idea how large the internet is. This is a problem that is so ingrained with automated filters that it has its own name - the Scunthorpe Problem - http://en.wikipedia.org/wiki/Scunthorpe_problem It is an inevitable consequence of this sort of system. Innocent sites will continue to be blocked.
As for your other point - it's pure conjecture. Furthermore, it will not stop kids accessing porn. When I was a kid we used to swap dirty magazines. Now they will be swapping memory sticks. And those are the ones who don't take 2 minutes to go to google to search for a way to get around the filter. For all the other ones it's business as usual.
The whole thing is a farce, designed to appeal to the Daily Mail / Think of the Children subset of noise-makers, while completely burying any hope of a rational debate.