Corsi isn't perfect but we're talking about players who are erroneously bad at it.
Goals aren't perfect either, but if you can't score 10 goals over a large sample, you're probably bad at scoring goals.
A quick note:
You say "When someone only scores 10 goals, he's not very good", yet Fowler isn't among the league's worst in Corsi metrics, and he hasn't been since his rookie season, when I think everyone can agree he was atrocious defensively without having to look up his stats to find a measly little 41 in the CF% category. He hasn't been plus, but he's hung within 2 percentage points of 50 for the most part.
Also, I'm including all defenders with over 100 minutes in my sorts when I'm looking at advanced stats for the purpose of this post.
Most people using advanced stats as a primary means of evaluation typically laugh at people for using pure points or goals as a means of offensive evaluation, primarily because there's plenty of factors that can change or effect goal scoring.
There's obviously the Cheechoo effect, and its inverse (in other words, quality of linemates). There's time on ice per game, which is countered by using G/60 and Pts/60 instead. (which in themselves are better than their traditional counterparts but still fail to live up the measure of a perfect stat) There's player health/condition. (whether they're 100% in prime condition, or playing through a nagging injury) There's cold and hot streaks. There's coaches, in whose systems player
x might play horribly in. There's player chemistry, meaning two or three players who might not be elite players offensively could put up good numbers, as well as its inverse. (players who don't mesh well together obviously won't perform up to expectations)
Corsi is much the same way. Too many people look at someone's HERO charts, see someone in deep **** in the possession category, and think "Oh, he sucks", without looking at context. Let's evaluate Fowler using the above qualifiers.
Cheechoo effect/quality of linemates: Inverse. This is the big one. Fowler has spent most of the year with Kevin Bieksa. Fowler, a player whose talents lie in moving the puck out of the zone (you should know his zone exit percentage is among the league's best) is tied down to one of the slowest defenders on our team in Kevin Bieksa.
TOI/Gm is mostly irrelevant since we're evaluating a percentage stat, not a flat number. Player condition is an unknown for the most part since we don't know everything that goes on in the trainer's room but I think that it's safe to assume Fowler, at least for the most part, was fine last year.
Hot/cold streaks is rather irrelevant. The Ducks obviously were mired in a huge cold-stretch earlier in the season, although their shortcomings were more on the offensive side as their possession numbers were solid, so we can probably ignore that.
Coaches/systems is another, but the main effect of this is once again drawn back again to Boudreau's strange obsession with giving Bieksa ice time, which usually has been next to Fowler this season. I don't think Boudreau's system itself really limited Fowler, although Fowler's been deployed in a shutdown role with a defensively unstable partner, which I can't imagine makes a good impact on his stats.
Player chemistry is another big one. Watching the two play, it's quite obviously that Bieksa and Fowler are not matches for each other, for reason's I've already noted. On a separate note, I'll also bring up the Lindholm-Manson pairing, since it's essentially a polar opposite. Neither are Norris-caliber players, yet it was our best pairing and IMO one of the better ones in the NHL.
Also, talking about anomalies that using Corsi produces, Kevin Bieksa somehow was over 50%; better than Fowler. Talk about Fowler's defensive prowess all you want, but Bieksa is absolutely not better than him. In fact, our 3rd best and 4th best players in CF% are Clayton Stoner and Kevin Bieksa, respectively. (1 and 2 are Lindholm and Manson) Stoner isn't awful. Bieksa wouldn't be bad if he was played as the 6th/7th defender he is, but he was god-awful this season given so much ice time. Korbinian Holzer was 5th. Once again. Not our 5th best defender. Still, none of them are better than Despres, Vatanen, and yes, Fowler.
EDIT: I should note that there's a general trend that shows that players with high levels of defensive zone starts tend to have lower CF% values. The highest-ranked defenseman to have a CF% over 50 was Ben Lovejoy, 17th in defensive zone start percentage, with a 51.3 marker. Cam Fowler doesn't exactly get shafted by the amount of defensive zone starts he gets, but he is placed fairly high, at 71st, and has the highest DZS% out of anyone on the team outside of Shea Theodore. (who had a limited showing) Naturally, players that get started more in the defensive zone are more apt to get outshot, though through no fault of their own. Cam Fowler's CF% of 48.8 is about around the mean of the people with comparable defensive zone starts. Also, Cam Fowler gets the 25th-least offensive zone starts out of anyone in the league, and once again a general trend is shown between players with low amounts of offensive zone-starts and CF% values. The lowest-ranked defenseman in offensive zone starts to have an over 50 CF% value is Marc-Edouard Vlasic, who gets the 15th-least offensive starts out of all defenders in the league.