Excessive camber (either positive or negative is a bad thing - and it's more than excessive tire wear. When the tires lean, they develop a "camber thrust," which pushes on the car. Tires with negative camber push inwards toward the center of the car. Tires with positive camber push away from the center of the car. Think about what happens when you lean a motorcycle. As long as both tires lean equally, the forces offset each other. However, if the car hits a bump that causes one tire to lose some of its traction, that camber thrust will push the front end away from the straight and narrow. Your car can easily make a quick turn in a direction you didn't intend.

If you have negative camber, you need to move the ball joint away from the center of the car (Sorry, Richard, I think you had it backwards). That's not easy. It could take a longer A-arm, or moving the A-arm mount outward. Unfortunately, if you move it a lot, that may have other effects.