In my opinion, the pin properties are most useful for things like power distribution and digital logic. For your specific case, I would argue all pins of a transformer should be labeled passive, as the component itself does not care about inputs or outputs (though the designer does!).
Although an argument could be made for labelling the input winding Power Inputs and the output windings Power Outputs. Whether this is useful for detecting errors is doubtful. And you’d probably end up putting a PWR_FLAG after the fuse and/or connector. The pins are passive in the library because the symbol doesn’t distinguish between the windings, or in fact between power transformers and audio transformerts; it’s just a generic transformer.
