The question should have been phrased "play by yourself" instead of "play with yourself" unless you always start games as both Player 1 and Player 2 and compete against yourself. Otherwise, the "how often do you play with yourself" question is a bit personal, isn't it?
I'd gladly socialize with a buddy while we take our turns playing one pinball machine. This was an efficient way for us to learn new games when we found them on location, since the one playing was concentrating on keeping the ball alive while the other person could watch the playfield and dot matrix display and tell the player what ramps or targets he should be shooting for during different modes. But once we each got a few games in our own homes few are patient enough to wait a few minutes between turns on a machine. My best friend is so impatient that if I'm playing well (and therefore taking too long for his liking) he'll fire up another machine down the row and start playing a game on it. Then when it is his turn again on the machine I've been playing, I have to wait for him to finish the ball he's playing on the other machine before he'll come back over and take his turn on the first game.
Frankly, wizard modes such as Rule the Universe (I don't know who is voting for me, but I like getting re-elected) and Lost in the Zone are reached easiest when I am playing pinball by myself and not distracted by socializing or taking turns.