If Bob creates a fresh address ...
That's the point. If.
Currently, it looks like, there are three types of transactions. People using the satoshi client, special institutions like miners, Mt Gox, exchanges and stuff - and a rest consisting of "strange" transactions directly fed into the system.
I am not interested in what could be done with the current system but what IS done with it. I have not yet reached all conclusions, but you can see quite a bit more in network analysis than I feel comfortable with. That's why I am concerned.
I hope, if I have a bit of time and in a few weeks, I can be more specific.