  In this letter we revisit the Standard Model predictions for
$\mathcal{B}(B\to K^{(\ast)}\nu\bar{\nu})$ and discuss the opportunities that
open up when combining its partial decay rate with that of $B\to
K^{(\ast)}\ell\ell$. In the Standard Model a suitable ratio of these two modes
can be used to extract $C_9^\mathrm{eff}$, which is essential for a reliable
phenomenological analysis of the $B\to K^{(\ast)}\ell\ell$ angular observables.
The same ratio also proves to be more sensitive to the presence of New Physics
in many plausible extensions of the Standard Model. We also suggest that the
separate measurement of $\mathcal{B}(B\to K\nu\bar{\nu})$ for high and for low
$q^2$'s can be helpful for testing the assumed shape of the vector form factor,
because the lattice QCD data are obtained at high $q^2$'s, whereas the low
$q^2$ region is obtained through an extrapolation.
