In general, I prefer the sound of more than one instrument and/or more than one voice to either an a cappella vocal, a straight single-instrument instrumental, or a solo vocal backed up by instrument(s).   It tends to make for a fuller, richer, more interesting sound.

But there are notable exceptions. Some ballads (whether folk or pop) seem to tell the story better as a solo.

FWIW, it seems to me that popular recordings (in most any genre) seldom have one single voice either a cappella or with only one instrument (unless it's a piano).   Even "solo" artists usually have vocal backup either by other vocalists or by overdubbing their own voices in places.

There seems to be widespread preference for a mixture of sounds, and duets or trios or backup vocalists can provide that.