One case from a science collaboration member: they would like to (as I understand it) forward model supernova light curves on the direct images, not the difference images. To do so they need astrometric/photometric transformations between various pairs of images, which their past experience suggests can be more precise if the transformations are computed directly between the images rather than each being calibrated to an absolute frame.
Totally switching gears: there are almost certainly dwarf galaxies that will be discoverable in a single epoch of LSST imaging. The standard way to find them is by making appropriate color cuts on the source catalogs and find overdensities of main sequence turn-off stars or giants. Because there are strong incentives to be the first person to discover the dwarfs, it is very likely that users will not wait for a data release and will instead try to download and photometer images on their own if source catalogs are not available (this happened with DES).