Effectiveness
I wonder if anybody has published research yet into the effectiveness of these blocks? We've been doing it in the UK for a little while now, but bypassing them is trivial. Site mirrors are everywhere, and the technologically adept (i.e. not me) can always use VPNs. That doesn't mean people will, however; some will give up remarkably easily, just as some will go to remarkable lengths to carry on as normal. I'm left deeply dissatisfied with the numbers game; how many are actually deterred?
Yes, siphoning might help the "leeches" to access the content. Let's ask editors from Australia about the situation.
I believe that even if in the short term these blocks will be effective, the internet will find a way to route around them.
Remember that the first file sharing technologies didn't rely on web sites and where not susceptible to such blocks. I expect that if push really comes to shove, we'd see something like that (only much better) come back, and torrents on the web will remain the exclusive domain of Linux distributions.