You really need to look at the spell before dismissing it. Heat metal also imposes disadvantage on the enemy while they're touching the object. If you cast it on armor, you get the 'just damage' as a bonus action every round AND impose disadvantage on all of their attacks. If you cast it on a weapon, they have the choice of either dropping the weapon or taking disadvantage (and damage every round when you use your bonus action) for the rest of the fight. That's a huge ability to weaken an enemy, especially a 'boss type' enemy. And for added goodness - while there's a con save listed, it's not a save against the better effects of the spell; if you make the con save, it just lets you drop a weapon without taking damage, it doesn't let you avoid the disadvantage effect.
Giving an enemy disadvantage on all attacks for the cost of a 2nd level spell slot and concentration is really powerful - sometimes it will cripple a major enemy, and at higher level it's nice for reducing damage from the 'some bandits attack you on your way into the dungeon' encounters.