You are probably right that the ruler would be more useful, if you could change the ruler’s Measurement Units via a context menu.

But I must confess that I never use the ruler.

By The Way:

You can change the measurement units “on the fly” by typing in a number in one of the measurement dialog boxes followed by
e for EVPUs (or evpus, or EVPUs)
pt for points (or pts, or points) 1pt = 4e
s for spaces (or sp, or spaces) 1s = 6pt
p for picas (or pi, or picas) 1p = 2s
i for inches (or ", or in, or inch, or inches) 1i = 6p
c for centimeters (or cm, or centimeters)
m for millimeters (or mm, or millimeters)

Note: When you’re overriding the global setting in this way, you even have an extra unit option (millimeters) not available in the sub-menu for Measurement Units.