1. I have found that 90% of the time the discussion on HN is valuable
even if I do not read the linked article. Often this is the case even if the majority of commentators do not read the linked article.
There are enough HN commentators who have subscriptions to for-pay sites such as the NYT, the WSJ, the Economist, etc., so that someone usually quotes the main points of the article in their to provide context for their comment here.
And there are enough smart and knowledgable commentators on HN that the comment thread often goes off on related matters that were not even in the article.
2. In the case of most journal articles like the one you give as an example, they are usually available for free. They just aren't free on the web. You might have to visit a library that either gets the printed journal or has something like a JSTOR subscription.
3. I'd rather be well informed even if I have to spend a little money to be so than be ignorant for free. If I see that some particular paywalled source gets regularly submitted to HN, those submissions regularly generate interesting and useful discussion, and from the comments of the people in that discussion who have access to the article I can see that it was a high quality article, that provides me with valuable information as to what paywall sources might be worth paying for.