It's too easy to miss upshift warning in 1st gear, and speed grows too slow for it to be of much use over basic rev counter in top gears. I'd put it as some max-100rpm and leave it there, without using/relying on it. Without audio beep, to not annoy one when gears need frequently switched. Some flashing in peripheral vision may help getting used to/feeling nearing of redline/acceleration cut. Of course i don't intend actually looking at it/monitoring it.
As for gear indicator .. there is no harm in having it but of course not much use for it either. It's displayed as extra, not instead of other readings. As for need - in most cases driver knows/remembers/feels better which gear currently in, and as Ade wrote, one can drive more economically then what it suggests. It may have use as extra information if one takes video from inside of the car, to ease other seeing what gear it is. What i hate though, that gear number changes only with big lag, when one depresses clutch, not when actual gear switch happens.
Edited by: church