If the old oil sits in the bearings much of it drains off during extended periods of time. The residue often stay on the bearing surface. Next start up it shortens the life of the bearing somewhat.
Also, if you leave the old oil in full, "acid" (it's not real acid) that is formed goes to the top. So you can get some slight etching into the metal at that contact point. I suspect that in antique cars they are talking about a longer than 1 winter storage time but am not sure. Of course if there is water from condensation in the oil yeah you can rust out the pan.