In my opinion, a manual shift car - especially a high revving one, needs a tach. For most people running an automatic transmission, the tach is superfluous... that said, I 'like' having a tach, but don't feel the necessity in an automatic trans car to have one.
I DO feel that having oil pressure and water temp are a necessity, and volts or ammeter gauge is desirable.
I think Chevynuts' earlier point re preferring analog gauges for most displays, but accepting a digital readout for speed is a good point, and perhaps the aftermarket gauge manufacturers could consider that.. a digital display is also acceptable for a 'Volts' gauge. and even for a fuel gauge and possibly for temperature. In fact for situations where the instruments are driven by digital inputs, why not display the digital as well as the analog?