Breeding for tameness, in my opinion, in regard to ASF feeders is strictly for convenience.
Calm tame feeders are easier to deal with than one that is frightful or aggressive.
Luckily my ASFs aren't aggressive with or without babies present. But regardless, I still would rather not have to worry about getting bitten or attacked when I stick my hand into their tub.
Being protective and defensive is a great survival trait in the wild. And ASFs are hardly domesticated. However, they don't need that kind of behavior here in captive breeding programs. Not to mention, I personally do not want a defensive animal in with my snakes. Best live feeder is a calm one.