Well yea on a 911. A "water temperature gauge" on an air-cooled car wouldn't tell you much!
OK, but what do you do with that information? Pull off to the side of the road until the oil warms up? I'm just not getting what action you would take based having both coolant and oil temp gauges.
If you build a really tight engine, like a race motor, oil temp is helpful to determine when you can use max revs and max throttle. If you use a lot is sustained high rpm and lots of throttle, oil temp can tell you when you need to ease up for a while. One of my cars does not like sustained operation above 5000rpm (more than 10 min and oil temp starts creeping up), and therefore needs an oil cooler.