  Linear spin wave theory shows that ring exchange induces a quantum disordered
region in the phase diagram of the title model. Spin wave spectra show that
this is a direct manifestation of competing classical orders. A spin liquid is
found in the `Goldilocks zone' of frustration, where the quantum fluctuations
are large enough to cause strong competition between different classical
orderings but not strong enough to stabilize spiral order. We note that the
spin liquid phases of $\kappa$-(BEDT-TTF)${_2}X$ and $Y$[Pd(dmit)$_2$]$_2$ are
found in this Goldilocks zone.
