Melding doesn't increase item level. Item level is needed to participate in content, melding is not.
But it does increase one's stats, therefore an increase in damage done. There's no good reason not to meld since materia is practically thrown at people for just about everything. Obviously it comes down to personal choice, but the fact still remains it is simple gains for minimal effort.
As far as duty roulette, it only matters in Expert, level 80 trials, or the Nier and Eden raids. Anything else, your gear is synced and materia doesn't work.
It increases sub stats but cannot increase sub stats beyond the ilvl cap of the item. Many gear pieces have sub stats at ilvl cap or close to it without materia. Substat bonuses aren't as inflated or impactful as some people have been implying and tbh make pretty much no difference at all in the end outside of attempting to speed run something that isn't ilvl synced.
10% sounds sus but it's been awhile since I looked at the numbers so lets take that at face value. Your 10% comes with a proviso. You'll get that kind of throughput if and only if every piece you're wearing is BiS. If you're rocking full augmented Cryptlurkers you're not going to get there. If the party you've been matched with aren't synchronizing buff cycles you're certainly not going to get there.
And this all assumes you've slotted optimal melds. There are resources out there that tell you what to slot but only if you've got very specific sets of gear.
Telling people to "just meld" is far more likely to get "whatever" tossed in there. That Tenacity is gonna work great on your RDM and don't forget to load up on Piety for your SAM.
A five minute meld job is not ever going to get you 10% throughput. You'd be lucky to get even 1%. Better than that would require a spreadsheet and if I was into that I'd still be playing EVE.
I'm not anti-meld. I won't step into a sync'd EX or higher without proper melds. I don't bother with the rest because what little difference they make simply isn't worth the effort.
You can tell every single job in the game to at least meld crit>dh.
Completely unoptimized melding with that basic priority is still ~6.5-7% increase on a straight Aug. Cryptlurker set.
For comparison, spending the time to farm tomes and an upgrade item to upgrade from a 510 exarchic piece to a 530 tome piece would be a 1-2% gain, despite taking significantly more time and effort than slotting some melds.
As I said previously, ignorance about melding is completely understandable. Being informed and arguing it isn't worth it is weird though, particularly when most people who don't meld still seem to be willing to do expert roulette, hunts or nier raids for other forms of gear upgrades.