Actually i find somehow this new system more benefic. I understand that in the old one, the symmetry actually changes the object topology wich is a problem in a more parametric way of work. If that object has somekind of pose morph target, in the old system, that would be a major issue, because it would change the object topology under the symmetry object. In your first picture the image stays original and the symmetry actually creates a new mesh. I think that is a good idea in order to preserve original topology. Another option is to activate scene symmetryze tool and work from there. Since it is a tool and not a generator, it will change the object topology as you prefer.
i have here some examples that might help
1 - mirror or symmetry (option for modelling)
2 - activate scene symmetry
3 - scene symmetry options
4 - symmetryze and options (this one is probably the one you like more)
5 - symmetrize selection options
6 - symmetry generator tool
cheers