  The workhorse instruments of the 8-10m class observatories have become their
multi-object spectrographs (MOS), providing comprehensive follow-up to both
ground-based and space-borne imaging. With the advent of deeper imaging surveys
from, e.g., the HST and VISTA, there are a plethora of spectroscopic targets
which are already beyond the sensitivity limits of current facilities. This
wealth of targets will grow even more rapidly in the coming years, e.g., after
the completion of ALMA, the launch of the JWST and Euclid, and the advent of
the LSST. Thus, one of the key requirements underlying plans for the next
generation of ground-based telescopes, the Extremely Large Telescopes (ELTs),
is for even greater sensitivity for optical and infrared spectroscopy. Here we
revisit the scientific motivation for a MOS capability on the European ELT,
combining updated elements of science cases advanced from the Phase A
instrument studies with new science cases which draw on the latest results and
discoveries. These science cases address key questions related to galaxy
evolution over cosmic time, from studies of resolved stellar populations in
nearby galaxies out to observations of the most distant galaxies, and are used
to identify the top-level requirements on an 'E-ELT/MOS'. We argue that several
of the most compelling ELT science cases demand MOS observations, in highly
competitive areas of modern astronomy. Recent technical studies have
demonstrated that important issues related to e.g. sky subtraction and
multi-object AO can be solved, making fast- track development of a MOS
instrument feasible. To ensure that ESO retains world leadership in exploring
the most distant objects in the Universe, galaxy evolution and stellar
populations, we are convinced that a MOS should have high priority in the
instrumentation plan for the E-ELT.
