I think that is a great idea, a tach and a temp gauge instead of a second clock that is not synchronized with the digital clock.
I am not crazy about the recent move in many European cars to remove the temp gauge (MINI, BMWs, smart, etc...). Without an oil temp gauge, water temp is a next best proxy to the oil temp. It is nice to know how the warm-up is progressing before reving the engine up, or if the engine is running hot while not technically overheating. An idiot light is a throw back to the 70's. At least in a smart, we can call it a move to reduce cost. But there is no excuse for not having it in a car like a 3-Series (I'll trade the fuel econ gauge for it any day).
Perhaps the creative aftermarket vendors will come up with a set of larger gauges (the stock gauges are almost too small given how far they are from the driver) for the smart. Ideally, full face tach on the left, and a combo gauge on the right (water temp and voltage). It would be even better if they are VDO gauges.