Strålman contributes with a few diffrent things than Girardi.
Overall, I wouldn't rule out that Strålman is better than Girardi, and that a McD-Strålman pairing would be better than a McD-Girardi pairing. Girardi is turning more and more into a "stay-at-home" D and I mean, just look around the league, those aren't that extremely important nowadays.
WITH THAT SAID, how you defend on a 3 on 5 PK, how you defend on any PK, how you defend 5 on 5 when really pushed back -- of course -- also matters. Those shifts count to, and there is no way to avoid them over 60 minuts and 82 games per year + the POs. The advanced puck possession stats don't include aspects like that. Hence, I do also think that those stating that the stats shown is a bit too black and white has a point too.
I mean just for example, you can't have a team with all players that are really good 80% of the time and very avg 20% of the time, its really valuble to have a line/unit that you can put on the ice and always knowing what you will get from it. You need to be able to impact the momentum swings of games and what not. Sometimes you want that unit to be able to step out and really win momentum for you, sometimes you just want a unit that can go out and not be scored against.
There is more to Girardi's game than how he impacts the puck possession of his team, and even goals scored on him or not and chances created.