That's one option. You can also google the title or URL of the article (this is the most common workaround); or you can search the comments for the word 'paywall'; or you can purchase a membership or subscription for the paywalled site; or you can skip reading the article.
So all I need to do is try every possible option? And even then it may fail (scientific journals, newspaper archives, etc)?
The links are just huge wastes of time. A prominent tag attached to the article would be ok, but in the absence of any other feature to avoid these time sinks, it makes sense to flag the articles to save others from additional wastage.
This site has a graph that say 55% of visitors to gaming sites use adblock http://contently.com/strategist/2015/07/10/why-adblockers-sh... . I would assume hacker news visitors would have a similar number. That would be an interesting thing to measure. Someone who gets to the front page of HN should measure what percent of people with a HN referer block ads.