I think PM’ing definitely has its place in AM work. Personally I try to take another approach when I send “cold call” PM’s.
First off, I will never just send messages blindly to random people. What I like to do is read a lot of posts from an affiliate, really see who they are and try to offer a good suggestion or helpful thought that I think may help them or their site. I do add in what program I am from, but really do not try and sell it to them. If the affiliate is looking to add another program, and likes your advice and helpful nature you will get way more responses then just blindly spamming. If you cannot personalize a “Cold Call” how are you ever going to make a good partnership with the affiliate after they do sign up. Overall some of my best affiliates and partnerships have been formed this way and I have yet to hear a negative comment about my PM’ing.
Just my 2 cents 