Actually, it's probably worse than nothing.
I don't know about those anti-P2P people, but when most people set up a honeypot or honeynet, they typically do it from IPs that aren't easily traceable to themselves.
So in other words, you're blocking 30% of the Internet, and likely missing the people you're trying to protect yourself from. So all it does is slow you down. So, like I said, it's probably worse than nothing.