I do that to discourage people from sitting next to me as opposed to sitting next to someone else. if I don't have a bag I'll sit across both seats. But if there are no other seats available I don't do it.
I remember the days when drunken Scotsmen, often with some dried blood on their clothes and skin, and always carrying cans of McEwan's, would get on board and make a beeline for me regardless of all the other empty seats. Not seen one in decades now but old habits die hard.