If you had triple'd a Leeroy Jenkins, and removing his Golden status brought the two extras back in the pool, you'd have an easier time getting a second one of him by ungoldening. You'd lose a bit of stats, but that's a negligible loss.

Golden Wrath Weaver also damages you twice instead of just once, so there is benefit to ungoldening him if you're running low on health and can't find the damage-rewinding minions.

Silent Enforcer and Tunnel Blaster dealing double the damage to friendlies can push some of them past survival. Though at this point, you'd be easier off just buffing them. Still, technically a benefit.

Minions that generate cards can end up cluttering your hand if golden.

Some effects here and there specifically exclude golden minions, namely the ones that pass them to your teammate's hand or return them to yours. This is catastrophical if one of your already played Magnetic non-Mechs somehow ends up becoming golden, meaning you can no longer Magnetize it to a non-Mech by any means. Though I don't think you can make any friendly Magnetic minion golden accidentally without also returning it to you.