IV's are free and so easy to get that any reason you can come up with for not using them just seems like an excuse to be lazy. Just try saying it out loud:

"Nope, I'm going to turn down a free upgrade to character strength that only requires a few minutes of my time every few months"

That's what most of the people in this thread sound like to me.

Maybe once upon a time when you could only get good materia through extensive spiritbonding and praying to the RNG gods would not melding make sense, but at this point in the game where IV's are so easy to get that SE may as well cut out the middleman and mail you some once a day I can't fathom why anyone would refuse to at least use them on their main character and class.