Well, the only standard gauge you can check is the temperature. If you lose fluid, you will be measuring air. So you can monitor the gauge all you like and it won't show you have lost coolant. By the time it shows overheating, it is too late.
Coolant level warning is cheap insurance, especially if someone else (like Mrs Spud :wasntme:) drives your car and wouldn't look at the gauges from one week to the next!

