Why is molality per mass solvent rather than per mass of solution? Knowing the moles of substance per mass of solution would be useful. Pour some solution and you'd know how much substance you have. Why is molality defined not this way, but as moles of substance per mass solute instead? It seems like this unit was engineered to have a value similar to molarity rather than to be maximally useful.

Why was molality defined this way? Is there an advantage I'm missing? This measurement seems easy to prepare but more cumbersome to use, and I can't see why that would be an advantage.
Molality $b$ is defined its way because being proportional for the given solvent to the solute/solvent molecular ratio.

Molarity or molar/mass fractions or molar_amount/solution_mass are not proportional to it. 

The boiling and melting point shifts are proportional via ebullioscopic/kryoscopic solvent constants to molality.

$$\Delta T_\mathrm{b}=K_\mathrm{e} \cdot b$$

$$\Delta T_\mathrm{m}=K_\mathrm{k} \cdot b$$



The number of solvent (s) molecules per 1 solute molecules 

$$\frac{n_\mathrm{s}}{n}=\frac{m_\mathrm{s}}{M_\mathrm{s}\cdot n}=\frac 1{ M_\mathrm{s}\cdot b}$$

If we provide as the input the solute mass and solvent volume and density, the above molecule ratio is 

$$\frac{n_\mathrm{s}}{n}= \frac{m_\mathrm{s}\cdot M}{M_\mathrm{s}\cdot V \cdot \rho}$$