I think you're missing the point - they're relevant but they're not quantifiable, so making strong claims based on their value is unreasonable.
No, I hear claims like that all the time regarding basketball stats from newbie basketball fans. They look at things like points scored, rebounds, assists, steals, blocks, and then when someone brings up things which are much more difficult to quantity as a number (but which have real impact on the game for anyone who watches the game) like "ability to create your own shot" and "harassing defense" and "get's into passing lanes" and "always in the right spot" and such, they ignore them because it's harder to put a number on them.
And yet, those types of things existed in baseball, and people did the same thing there, until finally they WERE able to put numbers on them. And things changed - drastically - in the entire sport. Because those formerly-difficult-to-quantify things were having a huge impact on the game all along, and most people knew it by simply looking but they were having a hard time putting a number on it. And now, it even arguably changed who won the world series.
The ability to move more than your foe, to take advantage of cover and concealment and the ability to hide and move silently, these all are quantifiable things. You could calculate the percentages for how often it comes up in an average scenario (using a statistically relevant number of published scenarios or something), the average decrease in damage taken, or the average increase in damage done to the foe over time, and the ratios of damage done to damage received, etc..
It's just that they are harder to quantify than average DPR from a long sword, or armor class. But everyone who plays the game sees the power of those other things, and knows they are having significant impacts on the game.
It is perfectly acceptable to say "I might not know what impact this has on average damage, but I know, and everyone who plays the game knows, it has a huge impact on the game quite often". That's not an unreasonable thing to do.
The unreasonable thing to do is to pretend that, because you're having a hard time pinning a number on an ability, that no claims should be made about that thing concerning the game.