Why water should not be soft for drinking purposes?

Solution-:

Soft water is plumbo-solvent and it attacks lead used in plumbing as:

2 Pb + 2 H₂O + O₂   ↔ 2Pb(OH)₂

The solubility of lead in water is diminished in the presence of    HCO^-₃ ions.  

A partial softening leaving a residual total hardness of about 85ppm CaCO₃ equivalents is generally adopted for drinking purposes.