When some voices approximate the sound of an instrument such as Ella Fitzgerald, Bobby McPheron, Al Jarreau or others, they can do it quite convincingly but not to the point where it replaces the ability of the instrument in the hands of a master musician but their voice then becomes their instrument.
I advocate, kendall, that every good musician's instrument is his/her "voice" and carries that uniqueness in phrasing, tone, rhythm, breathing etc. There is no disputing that Miles Davis' angelic horn was his unique "voice" when he played although he spoke with a raspy growl.
You might ask which voice is closest to what instrument and find some interesting comparisons there. A lot has to do with the individual frequencies of either human voice or instrument which affect the timbre. It's interesting to hear any master musician play different instruments and they bring to each one their own "voice". They often (if they can) sing the way the play or vice versa.
Some folkies baffle me because they react to the "harshness" or "edge" of a saxophone but tolerate the nasal growling or belting of their fave folk singer.
Again, one person's taste is another's bitterness.