I wouldn't but a scooter or a motorcycle without one.
Knowing where the maximum torque (6500) and power (8000) are, it allows you to know if and how
well the scooter will accelerate when you're at a certain (higher) speed. As any DT300 owner will
notice, at around 55-60mph, the CVT "shifts" to the last position (gear) available, and wide open
throttle there will sometimes result in the bike accelerating rather slowly maintaining just 5500rpms.
That same tachometer allowed me to know what Dr Pulley sliders to get (with a lot of help from TANWare)
to be able to put the "needle" where I want it at full throttle, and that's anywhere over 6500. 14gr sliders
did exactly that.
Also, having the rpm readout makes me ride slower at higher speeds since I'm aware of how fast the
engine is turning (and how close to it's maximum it is). I don't like to ride at over 6500 on my DT for
long periods of time. That was at 68mph before the Pulley upgrade, it's now at 75mph at those revs.