Especially in the example given above, all that you know is that you've got some target on your TCAS. In Jerricho's example, that's not worth a thing. It could be anything, it could be totally different from the target Jerricho is trying to point out.
But why is that different to a visual contact, Phoenix_X? It's just a shape in the distance -- it doesn't have an id-tag attached. At least on TCAS I'd know its level and probably have a better idea of its relative motion.