How is that any different than checking engine oil levels, coolant, windshield wiper, or brake fluid levels?
Hell, Ford could add that into their service plans.
Because the oil separator will fill up completely in 1,000-1,500 miles depending on driving condition. Most service is 10,000 mile interval these days. Not to mention, most people don't even know where to add coolant/wiper/brake fluid on their cars these days, especially on the luxury cars where under the hood is just a sea of plastic covers.