Summary: Hard water is (slightly) good for you.
If you ignore the effects of limescale on your pipes, boilers, taste (hard water makes worse tea) and any indirect effects these may have on your health there is evidence that hard water leads to slightly lower rates of cardiovascular disease. The result comes from a BMJ paper in 1980 which sought explanatory factors for the different rates of heart disease in different parts of the UK. The paper is here.
The summary conclusion relevant to this question states:
for other factors cardiovascular mortality in areas with
very soft water, around 0 25 mmol/l (calcium carbonate
equivalent 25 mg/l), was estimated to be 10-15% higher
than that in areas with medium-hard water, around
1-7 mmol/l (170 mg/l), while any further increase in
hardness beyond 1-7 mmol/l did not additionally lower
So it might be more accurate to say the absence of hardness has a small influence on stroke and heart disease rates. And the effect isn't large, especially when compared to other known influences on these diseases. As the authors say:
...the potential risks from smoking, hypertension, hypercholesterolaemia,
and physical inactivity are far greater than the
apparent risk from drinking soft water;
Another effect of water hardness that is almost certainly no longer relevant is that hard water prevents the accumulation of lead in water compared to soft water which tends to be acidic and leaches small amounts of lead from lead piping (which was relatively common in some countries until the 1960s). An older BMJ paper from 1973 summarizes this evidence like this:
The present findings suggest that people living in soft water areas
have higher concentrations of lead in their bones than those living
in hard water areas...
the present findings indicate
a more generally distributed cause; the ingestion of small doses
of lead from drinking water over a long period of time is a
And they further elaborate:
... a concentration of lead which may be
harmless in a hard water may not be so in a soft water; in fact,
hard water is protective against both the pick-up of lead from
pipes and its absorption by the body.
Since lead piping has mostly been eliminated, this effects is no longer likley to have any significance.
So the direct evidence seems to suggest that hard water is (very slightly) good for you.
I have not yet found more recent research results on the topic so it would be interesting to see whether similar studies from other countries or more recent studies show the same effect.