Flametongue/Ice Brand

These seem like very good (top tier?) offhand options (weather main handing Naegling or REMA)

Ice good for cleaving, Flame for melee, the additional proc seems like a significant dmg boost, can it be modified by any blu spells? (buffs or debuffs)? or setting jobs traits? (Macc/Matk)

The 30 STR and WSC mod seem like nice boosts too.

Is there any reason (beyond picking __ for ___ because ___) not to use these?