SB, experienced singers usually don't like it, since it's harder to gauge what they actually sound like. And reverb muddles the clarity of the monitors.

But if a singer is having a bad night and starting to get frustrated, a very subtle smidgen of reverb tossed in (if you don't mention it) can lull them into relaxing. You know what I mean -- you get frustrated & tense, and it makes you worse, so you get more frustrated...