In the HN community, non-constructive commenting is pretty strongly discouraged and moderated, so the urge to say things that normally would make someone want to hide their comments behind an anonymous name is pretty limited already.
So if nefarious activity is already not your goal, it simply becomes a personal preference. Why hide honest questions and discussion?
The only exception I can think of is if you're not well versed in the various knowledge domains that are popular here and don't want to give that fact away. I don't think that's a good reason at all though to be honest. I've felt this community be pretty accepting of honest questions and thoughtful input as long as you are honest with yourself as to what you're area of expertise is and you've done due diligence to the topic. If you're guessing something, you are going to be called out pretty quick. Don't ask for easily searchable facts, it's a waste of everyones time. Ask for opinions, experiences, etc.
A number of us have screen names because much of the early population of HN came from Reddit, which is almost all pseudonymous (the founders started off with pseudonyms).
Even on a more mundane level, what happens to you when the industry embraces yet another terrible idea and finds a sourced comment from you criticizing it?
1. There are many well known people whose Hacker News user name clearly identifies them.
2. There are many not well known members with similar names.
3. Many people with cryptic user names have identifying information in their profiles.
4. Hacker News is not a community with a meaningful cultural distinction between old timers and new users. The green names seem mostly there to assist in orientation to community norms and spam recognition.