Yoshi-P removed elemental affinities on mobs because he felt it led to class favouritism in parties, specifically for boss fights - he didn't want players preferring to manaburn enemies with BLM just because a mob had a weakness to fire or ice.

And you only have to look to FFXI to see how elemental affinity has been abused in the past (the infamous Cure Undead exploit is a case in point), which only influenced Yoshi's decision.