My auto has a tacho on it - I look at it quite regularly. Then again, I used the tach in all of my manual cars which had it (and really didnt like the cars that didnt have one).
Yes you can drive by ear, but its not as useful for situations like...
- Youre watching the rev count when having engine problems.
- Youre engine braking and not exactly sure if youve changed too early
Its also nice to know that when my trans is set to 'PWR', exactly what RPM level its gear changing at - as compared to normal, for example - and use this to ensure you're actually driving efficiently...
I think all cars should have em, personally...