Picking a second melee over a really good Black Mage is just the playerbase being stuck in the days of physical damage debuffs. There's lots of mechanical advantage to having three ranged dps.

That being said, if you're a world prog or week 1 raider, you should be willing to play multiple jobs to fit your team and benefit prog.

If you're not playing at that high level though, then stop worrying about it and tell your group to die less and play what you want.

Easy.