Art Thieme, I think ya got it nailed! LOL

Here's why I sometimes prefer to sing a cappella:
- Instrument or instruments likely to drown me out
- (Singalong) - The group knows the words & melody but probably will be all over the place trying to play along on instruments
- I haven't mastered the kind of instrumental background that would enhance the song (e.g., I can't play an F#-diminished 7th-with a flatted 9th (or whatever fancy chord(s) I'd need)
- It's really a banjo song and I don't have one of those (and couldn't play it if I did)
- The instrumental accompaniment should be a flute and I didn't bring my floutist with me
- My fingers are bloody sore from playing for a couple hours already
- (Group singing) My harmonizing isn't limited by the chords the guitar or piano decides to play*

Here's when I HATE it when people sing with no accompaniment:
These all pertain to sing-alongs or other group singing
- The group has no idea what key we're singing in (and the first verse is loused up before we decide on one)
- The song leader has no sense of rhythm
OR
- The group can't or won't follow the timing of the song without some sort of instrument leading
-
- Some of the people singing don't really know the tune, so can't stay on it without hearing the chords



*This is especially important when just a few of us are doing harmony.   Way too often, when the guitars stick to a basic 1-4-5 chord pattern (for one example), it sounds fine for the melody but precludes a lot of really nice harmonies.