I do this too. I found it almost impossible to post on my account under my own name, on any topic about psychology, politics, economics, or diversity because I am too afraid of what I've said being taken out of context later, or used against me in ways I can't anticipate now. I found myself frequently writing a post, only to delete it shortly afterward, worrying that it was somehow too risky.
In today's climate, you can't really even propose a thought experiment or discuss a hypothetical without being accused of holding that position yourself, and criticized as such. I don't even really have strong convictions about most of the things I discuss, except the desire to analyze them rationally and objectively, and the willingness to challenge both conventional wisdom and new radical positions.
Even while posting under an anonymous username, I still find myself self-censoring because of the risk that my identity could someday be connected. HN administrators can certainly trivially connect me, since they see the origin of my traffic for my two accounts, and I have not taken pains to anonymity the traffic for this one. However, HN admins themselves like pg have been posting under alts (and probably still do, for the same reasons), and seem to support this.
I have been wanting to propose that HN or some site like it offer a feature where you can switch your post to "Anonymous Coward" later, if it proves to be too controversial. Or post as "Anonymous Coward", and later assign the post to your name if you feel OK about how the discussion turned out. I think this would be better than having people feel like they can't post at all, or that they need alts to post. Having an alt is kind of like Anonymous Coward, except that you can't claim credit for just a single post that you turn out to feel OK about later.
The thing is, I think, that humans "try on" ideas like they try on clothes. We don't necessarily mean everything we say all the time (unless we take great pains to ensure it). The public discourse seems to expect that people have their minds firmly made up about everything, and a completely firm viewpoint that can be understood and criticized, but the reality is much more fluid. Or to put it differently: writing something that you can firmly stand behind requires a lot of time and energy, and a bar of quality and thoughtfulness and judgment that can rarely be met for Internet forum comments. Sometimes I'd just like to have a conversation, without fearing that something I say can be taken out of context and used against me years later when random-topic-of-the-day becomes a hot-button issue.