If you ignore positionals then yes they do matter and they do make a big difference...
No one here is talking about literally getting yourself killed just to hit one positional it's really disingenuous to try and spin it that way.
No missing a positional isn't going to annihilate your dps, ignoring them will.
Count the amount of Shintens ( and possibly Kaiten depending on what you're doing ) you're missing and then add all of that potency up and you'll see that it actually is a big deal to ignore positionals...
And if you're the type of player to ignore positionals then quite frankly you're probably doing a lot of other things wrong too because you clearly don't care about making an effort at that point.
Ignoring positionals I'd say is indicative of a lot of other low effort and bad things too.