The BOSS project on SDSS3 is taking spectra of hundreds of thousands of quasars. This requires selection based on existing imaging. Some kinds of quasars in the redshift range of interest (around 2.5) are confused in the imaging we have with blue stars and redshift-one quasars. None of these types of sources is detected in any GALEX catalog, but the redshift-one quasars do have, on average, a bit of ultraviolet flux. Today Schiminovich, Hennawi, and I found that there is hope that even two-sigma GALEX detections might serve as a strong veto of redshift-one contaminants.
The (obvious, in retrospect) punchline is: If you care about improving samples contaminated at the tens-of-percent level, you can use one- or two-sigma measurements; they aren't reliable, but they are as reliable as the sample you started with.