Not being a professional cook, and having never used high end equipment, the "flat top burner" range is a mystery to me. A 12" cast iron grate over a round gas burner I totally understand. What's with the flat top? But I saw a line of very high end home range one of the options is a flat top in place of 2 burners. This was distinct and separate from the "grill/griddle" option. In photos I've seen these things as well, on a grander scale. Flat metal surfaces with pots and pans on them, with rings in them to expose the flames directly, but were usually left in place. I also saw this kind of surface on 2 of my grandmothers cooking stoves, her wood fired kitchen stove/oven, and the wood fired heating stove in the living room (living in the deep woods many things served double duty.).
So, is there a simple reason the high end places use the flat top over the open burners?