I think people should contribute, but I am also going to accept their assessment of things. As I think should apply in more places than just Hubski. If you do not think you are good enough to contribute something or attempt something, then fine - don't. The only thing that is holding you back is you.
Once you try, you can get an actual metric for whether your attempt was "good enough." But, assuming other people are capable of this line of rationale as well, I say maybe Demosthenes knows what the metric is for conversation in Hubski and so truly is able to measure his would-be comments against the "Hubski standard" (like the gold standard, eh?) and sees that perhaps what he would contribute is somehow "not enough." Although I would hestitate to say that the bar is really very high. I mean, what's the worst case scenario? You bring down the average on Hubski for a bit - that's ok, we'll recover. You make yourself look like an idiot? Bomb the account and start again, no one will ever know.
If Demosthenes is incorrect and it's only perhaps his fear holding him back, well - I have little patience for those that allow fear to run their lives.
All this being said, Demo, I encourage you to contribute, when you find you have something to say that you think is worth saying. I agree with 8bit - it doesn't have to be a paragraph.
You could always try one of our clubs, first, like the book club or the wine club. They are perhaps slightly more casual environments.