I wouldn't say it's more relevant than absolute points. Crosby's PPG was nice in that season but the objective is to help your team. What help is he if he's playing in only 1/4 of a team's games? And in many cases (not Crosby's) the extrapolation doesn't work because it never takes into considerations streakiness. It's hard to say what the player would have actually done over 82 games because...he didn't play those 82 games. It can be useful at times, no doubt, but as with everything you need to look at the whole picture, and for me, I'd only use it if I'm comparing a couple players in similar situations with a history of some kind of consistency. Not really caring about the guy averaging a point per game but can't seem to get into more than 50 games as much as the guy who seems to play 75 or so games and is integral in his team winning, even if his points are only .75 PPG.
Of course it is more relevant. It's referring to production on the ice. If he's only giving 30 points a year but is doing it in 40 games that's completely different then doing it over the course of 70 games.
If you want to discuss injury history, that's completely fine, but that's also a different topic. Stempniak has been fairly consistent over the course of his career when it comes to goal/point totals (bar a few outliers) from that, you can gather a general sense of the kind of production he will provide, points-wise.
To argue otherwise is just pure minutia.
When Gaborik had those bunch of years that he missed games with injury did people not consider him elite, absolutely not. They just called him out for being injury prone, but extremely productive when he's on the ice.
Stempniak surely isn't at the level of Gaborik, but it's similar scenarios.
And if you want to play this 30 point semantics card and not pro-rate it, then technically to get a true indicator, you would have to take into accuont his replacement for the games he misses, which would still come out to about 40-50 points per years due to the fact that Stempniak is missing those games and allowing someone else to play and put up points.
It's not as if him being out of the lineup is making the Rangers or any team for that matter play a man down.