I see where youre coming from now...
yes you could make that argument that it does work "through a range of speeds" with the fact that it will show rate of turn at varying speeds - and good enough to use in IMC...but if it showed up as an answer to a question on an exam or during a flight test, Id be weary at picking it without considering all the other options.
It will only show the proper rate at one airspeed though, as DFC stated. However, for say a C152 or PA28 the speeds they go are slow enough to ignore the error - with the error also depending on mechanical errors like friction, wear and tear, et al.
Remember too that (and I assume this is the same with the JAR's - correct me if Im wrong) when IFR your turns should always be at rate 1 or 30* of bank, which ever is the least, so above 230 knots you will always be 30* and below it will be rate 1.
But if you can get to 230 in 152 without becoming an unintentional parachutist...youre a better pilot than I.
