unless you are being charged on the basis of the reading, in which case it's possibly a ripoff.
Possibly, but not in my experience. I like paying by tacho, beacuse its very precise (in increments of 0.01 of an hour (36 seconds)) and also, taxying around airports doesn't cost me the same rate as cruising does, unlike many schools which charge by hobbs, in increments of 6 minutes.