fWAR stands for Fangraphs WAR for those who don't know. It uses defense independent pitching stats (in this case FIP) to calculate WAR. FIP includes only the events a pitcher controls: strikeouts, walks, hit by pitch and home runs. It is not an accurate account of the runs a pitcher has allowed, but the number of runs the pitcher would have allowed given average luck (balls in play and sequencing). Glenn argues that fWAR is more predictive of next season's WAR for pitchers, which isn't actually that surprising since FIP is more predictive of next season's runs allowed, but it's an interesting article.
The first part of the hypothesis—rWAR is better at describing what has happened—was not tested in this analysis and still is a point that is up to debate. I think the second part of the hypothesis was answered to some extent with this analysis.
There is a good deal of evidence that backs the idea that fWAR is better than rWAR at telling us what will happen.
I still would argue that WAR, as a metric, is supposed to be describe where wins came from on the field, as opposed to being a reflection of true talent level. But I am only one man, and there are many who still want WAR to be a true-talent metric. Based on fWAR's predictive/true talent qualities, there is definitely an argument and a place for the use of it.
I would argue the same thing (WAR is supposed to describe what happened). This is why I prefer rWAR for pitchers. Whether or not it's better at describing what happened is up for debate, but FIP does not measure what happened. rWAR uses runs allowed so one would think it would more accurate describe what happened. As for predictive value, I've always used FIP over ERA.