I was worried about this too but aren't anymore after dabbling in some endgame content.

At start most people played anything in AF and content at the time ifrit/moogle/DH were forgiving enough that medicore skill and gear was enough to clear them. As content gets harder and skill and gear start to matter more, most people will start focusing on fewer roles to play.

WHM that only leveled the job up for the crown won't be as reliable as an experienced WHM with melded and endgame gear.