Are there any general guidelines for modern practice about when vocal soloists should memorize their music instead of using a score? I've seen in many performances even of "standard" works like the Verdi or Mozart Requiems that soloists will use scores, but at other times—such as in recitals—they get used much less often? Is it just the need to coordinate with a team of soloists versus working with a single pianist that makes the difference? Is it the difficulty or familiarity of the work? Or are additional criteria involved?
One reason is dramatic. Just standing there in front of an orchestra and chorus all dressed up, with nothing to do with your hands, can be quite awkward. It's more acceptable to do a bit of acting in a recital.
Perhaps it should be accepted for soloists in an oratorio or similar concert to do a bit of gesturing, but there's not usually sufficient time to get all the soloists on the same page, figuratively. It would be odd for one soloist to do a bunch of gesturing while another stands with arms hanging.
It's also unlikely that all soloists in such a concert would agree to sing from memory. It would also be distracting if one soloist is using a score and another isn't.
Nobody wants to stick out, and the least common denominator is almost always to stand there with a score in your hands.
With enough rehearsal time, I don't think anyone would disagree, singing from memory is ideal. The problem is that in a professional concert, soloists may not have enough rehearsal time to be certain that they can sing from memory. A soloist hired for a community chorus concert, for example, might have one or two rehearsals with the orchestra and chorus. The soloist will almost certainly need to use the score during rehearsals to take notes, to refer to specific measures when discussing questions of interpretation, and so on. There might not even be a single opportunity to run the piece through without stopping before the concert.
For "higher-level" performances, such as with a professional orchestra and chorus, especially for better known works, it is more likely that vocal soloists will perform without a score. The soloist is probably paid better, so can devote more time to work on memorization before the first rehearsal, and the greater prestige of such a performance probably also encourages the soloist to prepare more thoroughly. In addition, there is probably more rehearsal time with the orchestra.