You're right that they do often show the local magnetic declination's rate of change (around the time of publishing), but even that goes out of date eventually.

You're right that they do often show the local magnetic declination's rate of change (around the time of publishing), but even that goes out of date eventually. For relatively short periods (maybe on the order of years or possibly even a decade or two, depending on the particular location), applying this rate of change may give a good approximation of the present day's magnetic declination value, but beyond that the errors can become unacceptably large.