Short delays (between 10-30 ms) will fatten up a sound and you won't hear a distinct delay. Still it can hide things (bad things) that may actually be more noticeable out front. That said to the singers, reverb can make things sound more pleasing and perhaps more forgiving. Some reverbs actually have a bit of pitch modulation going on which obviously make it a little trickier to find your pitch. IMHO reverb can make feedback more touchy, and too much reverb can make finding your pitch more difficult.
I always prefer to run the monitors totally dry. If I'm running sound from the stage I'll almost always run a little reverb, but no delay/echo due to the fact I can't easily tweak the timing of it while playing guitar. In other words longer delays work best with a soundman to continually tweak it in all timing wise. Once you get passed a slap back delay/echo it might only sound good IF you time the delays rhythmically. Some delay, or even longer echo can work but sometimes (for longer repeats) the timing becomes more critical. A little reverb in the mains, though the choice between a long, or medium length or even room verb depends on the music style.
