  We demonstrate that at relatively low mass accretion rates, black hole
candidate (BHC) X-ray binaries (XRBs) should enter `jet-dominated' states, in
which the majority of the liberated accretion power is in the form of a
(radiatively inefficient) jet and not dissipated as X-rays in the accretion
flow. This result follows from the empirically established non-linear relation
between radio and X-ray power from low/hard state BHC XRBs, which we assume
also to hold for neutron star (NS) XRBs. Conservative estimates of the jet
power indicate that all BHC XRBs in `quiescence' should be in this
jet-dominated regime. In combination with an additional empirical result,
namely that BHC XRBs are more `radio loud' than NS XRBs, we find that in
quiescence NS XRBs should be up to two orders of magnitude more luminous in
X-rays than BHC XRBs, without requiring any significant advection of energy
into a black hole. This ratio is as observed, and such observations should
therefore no longer be considered as direct evidence for the existence of black
hole event horizons. Furthermore, even if BHCs do contain black holes with
event horizons, this work demonstrates that there is no requirement for the
advection of significant amounts of accretion energy across the horizon.
