I rarely reply to a thread that has run this long but shall in this case.I was primarily a guitarist and singer intil I hurt my hand. Then I was primarily a singer until I retraned my fingers. Now I am still a singer but also a guitar accompanist. For the guitar, or banjo is more a tonal and visual aid. It neither detracts nor overshadows what I do. I find that I sing more accapella songs than before but I always sang that way as well.
Its the song that dictates the technique more that anything else. I will always sing "Rhue" " The Burning of Kingston" "Henry the 8th", "Spainish Johnny" and a host of others without an instrument because I feel thats what those songs need and how they suite my voice.
I think an unaccompanied song requires more preparation in many ways to sing well. With out the rhythym of the instrument, the awareness of breathing becomes more critical. Pronounciation does aswell. Vocal emphasis becomes more necessary for reasons of song dynamics.
Without the instrument staying in Key becomes more critical. Rarely does an unaccompanied snger start and end on a perfect tone.
Any one who says you musicianship ruins or detracts frim your singing either is very jelous or is very rude. Sing the song how you hear it, guitar or no.
Don