Skills for an Inferno hero.

I don't really consider Hellfire to be some super-powerful ability that a demon absolutely must enhance, and opponents absolutely must invest in protection against it.

I'm not saying it's a super-powerful ability, I'm just saying that demons take it almost always, and this additional damage can be quite unpleasant, and fire resistance against a demon is also very desirable because it reduces the damage from the Pit Lord's Fireball.

If the demon has Withering Flame, the effect of abilities is canceled.

Not quite. Withering Flame increases the damage from Hellfire by 50%, so if the damage was previously 100%, it becomes 150%. Fire resistance reduces the damage dealt by the fire element by half, so 150% divided by 2 equals 75%. Therefore, if a hero has fire resistance, and the demon has Hellfire and Withering Flame, then when Hellfire triggers, the unit will only receive 75% damage; if there is no Withering Flame, it will be 50%.