So, about to hit level 20

People get 2 because some ADCs need the extra AD to last hit caster minions under turret after 1 tower shot.
Though it doesn't always come into effect. (ex. how ahead you are, your support can set up those last hits if you're coordinated enough, you might play a champ that can do it w/o the extra AD.)

Its just general though, if she likes the extra sustain from the 3rd she can do that also.

That makes sense. I prefer knowing reasoning before I do something, so I might actually change to 2 knowing that. I'd have to actually try them in a custom and see the difference.