I've looked at the inside of a PT6 cowling intake duct, and been surprised at how "low tech" it is, especially towards the engine end. Apart from the turn in the duct at the inertial separator, the air at the end of the duct makes some fairly low-energy sharp turns into the actual engine inlet. I suspect that a bit of drizzle sucked through the intake (and the PT6) is not a big problem, but I also suspect from looking at the shape of the duct that if you fly in snow/sleet (or anything else on the solid side of "visible moisture") without the separators deployed, it'll collect in the duct pretty darn quick.