I don't drain mine monthly. Here in the Northwest I have never seen more than a drop or 3 of water in my fuel after draining. Usually after letting it set and settle out for a day or 3 the amount of water is negligible like grain of sand type stuff, or none at all. So after checking frequently initially, I start to check it less as time goes on. But depending on how much I drive, I'm going to do it at least every oil change.
I think this is just something that people do to stave off issues. Sorta like wearing a rubber vs the pullout method. The system, pullout method/trusting the WIF sensor, may work pretty well. But wearing a rubber/draining the water separator, is a little more proactive and might save you a ton a cash
Not a perfect analogy, but amusing.