The thing is, if you have 50 objects in a family, and say 25 of them use a behavior, it'd be much simpler/cleaner to
do this:
+Foreach Family1:
-> Behavior action.
or even:
+Foreach Family1:
+Instance has behavior (enables access to behavior's actions/conditions in sub-events)
-> Behavior action.
+ Behavior condition
->behavior action.
...
The reason it'd make it better is because if you need to add or remove objects to and from a family, you don't have to create all the extra events to do the same thing.
Sure, you can put the ones with the same behavior into a family and give the family the behavior, but that would just complicate things and introduce bugs and make writing the events counter-intuitive.
Try putting physics behavior on an object, and then put that object in a family where the family has a physics behavior and you'll see it get messed up and not work because the behaviors clash with each other.