Well this recent phishing attack is clear demonstration of the inherent dangers of homogeneity. Biologists and epidemologists have known this stuff for decades. Having given out my Yahoo password would have been much more disastrous if I was using Yahoo for my primary email address. Fortunately I use Gmail. Imagine the damage had I given out my Passport password. Egads!
Unfortunately I do use Yahoo Messenger. But I also use MSN and Skype. One password does not connect the bad guys with everything I use to communicate. But it is enough for them to do some damage. When you get an IM from a credible source, it is hard not to resist. It naturally brings your defenses down. A clever example of social engineering.