This is very uncommon.
No it's not. It's very commonly done to provide pre-separated pages to local silkscreen shops. Eliminates mistakes; simplifies workflow for people who often don't really understand color separation. Enables them to print their screen positives on non-PostScript printers.
Years back, in FreeHand, it was even easy to ensure that separation-specific custom halftone settings survived the trip.
Europe, Middle East and Africa