Earlier quoted context omitted.
For most purposes, it is de-anonymizing. There might be several famous people who share your name, but is there more than one person with your name who is currently an undergrad at Caltech studying CS? In most cases, a name combined with a little bit of context will identify an individual.
The difference, for me at least, is effort. There's a vast difference from if people can do a search for your name, and get page up and page down of stuff from every forum you might post on, vs. if people need to track down your nym's. In my case, my HN id is trivially easy to connect to my full name, but unless you go to the effort, these comments won't show up. My Reddit comments are still easy to connect to my ful…
There's nothing wrong with an individual choosing to use his legal name in his profile, but we should be weary of making that decision for others. Personally, I won't join any service that maintains such policy.