I think what they mean is. Melding gear just for the 2k gil or whatever the challenge log gives you is gonna be a very low income. Chances are you could just sell the materia and yield more gils than the challenge log would give you.
Melding just to increase the spirit bond speed also is a very low return on investment Because it still takes a lot to spirit bond stuff and for the most part gear is replaced long before the spitit bond is even completed meaning that you'd have to make the extra effort to go out and actually finish spirit bonding it. And for the return on your time and investment its just not worth it
So if you want to make cash from materia. The best thing you can do is literally just sell it on the markets...
Gear being replaced so quickly is one of the big reasons so many people just don't bother melding. And you cant blame them.
As I said earlier if gear lasted longer or melds carried across augments then more people would see it worth while.. but why meld a cryptbunkers set when your just going to augment it and destroy the melds. It's throwing potential money down the drain basically. Hence why so many people don't meld.
There is however one point Vyrerus raised earlier. That I'll admit even I hadn't realised is that tier 6 materia actually has better stats than tier 7. (All my 520 cryptbunker has t7 melds derp!!) And can be removed with 100% success apparantly.
So this then becomes an issue of player awareness Players don't know they can meld there stuff with absolutely zero cost or zero potential loss because they can get 100% of the materia they meld back. Although there is still the issue of gear lifespan. and the fact that players typically over power contnet on day 1. the newest alliance raidis i495. when players are typically 515-525 so already outgear the contnet