It's not the only way but I agree that the plate system is easier. You could press fit bosses into the slide after milling to locate the optic but locating those bosses would be challenging. If you screw up boss location your not just tossing a optic plate, your trying to fix a slide.
With the CZ Custom RDS plate system (I can't speak to the Optic ready variants direct from CZUB) you aren't sacrificing how low your optic gets mounted. Is it as low as possible? No, but practically it is low enough. My SRO sits maybe 1-2 millimeters over the slide. Plus I can swap out the optic for fixed sights whenever I want.
Cheers,
Toby