Ya, Malcolm. I can think of a few other situations where this may not be the best policy. When requested by physicians, we have done eluates on compliment-only positive DATs where we ID antibodies, showing that one can have a "false IgG negative" DAT in certain situations.
Anyway, in most cases we would repeat the eluate if, in the first place, we identified that an allo-Ab was present on the patient's cells. But as for initially negative eluates, if a repeat DAT is still positive but not stronger than the previous, we would not bother with another eluate. The idea being that if the patient is producing a significant amount of antibody, the DAT reaction would be stronger.