I don't know if there were any pain elementals in doom 3, but in doom 2, they always spawned 3 when they died unless you had too many pain elementals in the level already. When there were too many, they couldn't spawn any, before death or after,
but that's the only limitation on spawning that I remember. They also couldn't bite, so in that way at least they're harder than in doom 2.
You're right though, they are a bit easy, especially since all their lost souls tend to crowd around them. Lost souls get dangerous if you have to keep changing your field of fire to gun them down before they start gnawing on you. Maybe a change in their behavior when spawned by elementals would be a good idea: they spread out a bit before closing in, making them harder to hit with a narrow stream of fire.