Not common. From what I've read, each year, 50-80 people are killed on the tracks (either by hitting trains or touching the third rail), with only a handful being caused by somebody else pushing them onto the tracks.
You and everyone else of course has their own barometer of what is an acceptable tradeoff. I'm not trying to convince you in particular that NYC transit is a good or bad experience overall; I'm explaining why it is reasonable for someone to come to the latter conclusion.