Psy's particle effects have a collision modification to them so it can determine when an object is blocking the rain effect, preventing the system from raining indoors. In the case of Congo, I have a lot of indoor areas scrunched up around the outdoor areas, so being able to use Psy's particle effects meant I only needed to use 3-4 systems to cover my areas.
Plus he has a cheap version of it which doesn't crap all over performance as badly as sawmill's ones do.
EDIT: This is, of course, assuming Valve hasn't screwed around with the particles since the steampipe change. I'm still trying to put things together again since the SDK was broken.