  Although controversial, the scenario of microlensing as the dominant
mechanism for the long-term optical variability of quasars does provide a
natural explanation for both the statistical symmetry, achromaticity and lack
of cosmological time dilation in quasar light curves. Here, we investigate to
what extent dark matter populations of compact objects allowed in the currently
favored Omega_M=0.3, Omega_Lambda=0.7 cosmology really can explain the
quantitative statistical features of the observed variability. We find that
microlensing reasonably well reproduces the average structure function of
quasars, but fails to explain both the high fraction of objects with amplitudes
higher than 0.35 magnitudes and the mean amplitudes observed at redshifts below
one. Even though microlensing may still contribute to the long-term optical
variability at some level, another significant mechanism must also be involved.
This severely complicates the task of using light-curve statistics from quasars
which are not multiply imaged to isolate properties of any cosmologically
significant population of compact objects which may in fact be present.
