I also never do it, and fine people who do to be selfish and arrogant.

So many times I could not find two seats together because all those people put their bags on the chair next to them disallowing another solo traveler to occupy the seat thus leaving a pair of seats for couples.

I will leave a bag on the seat (but usually my jacket) if I need to get a drink or something to eat so that people will know that someone is sitting in that seat.