Probability isn't exactly my strong suit (suite?), but as stated before, that 20% of applied to both direct and splash damage, so that's probably somewhere around 40% to start a fire per hit already, and possibly a 4% chance to start two fires?
Correct me if I'm wrong about the above, but that already seems like an excellent chance to start fires with weapons that have either large clips or tons of particles launched per shot.
Also, isn't incendiary sometimes used on the mine launcher?