There are a number of conditions which require negative pitch. One of the more obvious is weathervaning. The Bell 206, for instance, weathervanes strongly, and in a 20 kt wind will stay into the wind without any pedal input at all, in an OGE hover. I've done it offshore, going from flat pitch up to an OGE hover and back to the deck with my feet on the floor, not touching the pedals. If you need to turn to the right, you have to put in a lot of right pedal, and it simply won't turn without it. Most helicopters will weathervane to some extent, and with a crosswind it can take a lot of negative pitch in the tail rotor to make the nose point where the pilot wants it.