Tier V materia is great.

Tier V Primary Stat materia is awful.

In all but a few cases ANY piece of gear you equip is gonna have plenty of Primary Stat. So INT V, DEX V, and so on are pretty much as much junk as Thunder resist V or Wind resist V. Arguably even moreso.

There are some exceptions: Tanks are wise to meld STR materia to VIT accessories, as it has a greater impact on their damage than Crit or Det. In some cases, healing jobs may want to meld VIT to accessories when taking on challenging raids for the first time, as survivability is very important for them.

For the most part, though, primary stat materia is junk. You should focus on melding Crit, Det, and possibly Skill Speed.