.. Everyday I also see people choosing an empty pair of cramped airline seats over tables occupied by one person - it is very normal not to want to sit with strangers.
Well, if you're alone, airline seats are loads better: at tables you do anything you can to avoid knocking/rubbing legs or knees with strangers opposite you, so you have to tuck your knees under seat or sit up straight. However cramped airline seats are, at least you can attempt to stretch legs.
On the other hand, if you know the person opposite you, it doesn't matter.
I'm quite short, so cramped airline seats don't bother me and I would always prefer them above a table if alone. My tall mate, with whom I travel, likes to look out of the window rather than play Angry Birds, has an unspoken code of preferences which we always subconsciously know, depending on if it's dark or light and if a table seat involves him sitting opposite or next to me. We just get on a train and look at the seats available, maximise our comfort without even thinking about it. I could draw a flow diagram...