Using such internal ports is a common way to model antenna excitation. Pro: it is efficient to model it this way. Con: as you already pointed out, this "ideal" model that doesn't account for common mode signals that might be exist between a coax shield and an external ground. However, to get those effects, you would need to model the antenna with coax of reasonable length and realistic ground environment. That's usually overkill. The typical approach is to use internal grounds in simulation, and avoid unwanted modes in the hardware by using proper baluns or ferrites.