I have actually filed DMCA complaints with Google, and some of them are republished on Chilling Effects. I believe that although the DMCA process itself is often abused, this research itself is flawed.
Let's look at a couple of headline figures:
"57% of notices [...] were sent by competitors": well, it's bleeding obvious, isn't it? If you steal my intellectual property, then there's a very good chance that you are in some way competing with me. If I were to rip off whole stories from El Reg and publish them on my own competing site, then El Reg would be perfectly within its rights to file a DMCA complaint.
"37% of notices to Google related to sites outside of the US, where different laws would apply": it is hard to imagine that this "research" could get it more wrong. It does not matter if the web sites are outside the US or not, because Google *is* in the US then it is bound by the DMCA. As a non-US citizen, I can file a DMCA complaint with Google about a site in any country whatsoever, and Google will (usually) remove it from their index if the proper paperwork is filed.
Fundamentally, the "research" makes a very silly mistake. If you file a DMCA complaint with Google, it does NOT remove the offending site from the internet. It simply removes the content from Google's index (unless we are talking about something like Blogger). It is NOT the same as filing a DMCA complain with a US-based web host, where the host is obliged to ensure that the content is actually removed from the site (usually by shutting the site down).
Actually, perhaps I am being a big harsh on the research. If you read past the attention-grabbing-but-flawed introduction, there is some interesting stuff there. But I am guessing that most people won't, and will end up with the wrong impression.