Instruments (and voices come in the same category) They have a range of widths. Voices, trumpets, trombones etc have a very, very narrow width. All their output bar a small amount comes from one location - They are in effect, mono sources. You can argue that in a singer, some of the sound comes through the body, usually the chest and throat (which it does), and brass instruments do produce small amounts of energy through the metal - but it's very, very small compared to the primary source. The widest instrument would be something like a church pipe organ - where the width can be huge. Even a grand piano can have a sound source that can be five or six feet wide. Other instruments have multiple sections, or holes, or extended vibrating bits that mean what comes from one side is different to the other.
If you wish to record the instrument then in a dead space, where you want as much instrument and as little room as possible in the mic's output then a voice is pointless recording with a stereo technique, unless you want the effect of the singer moving their head, for weirdness. The grand piano, on the other hand loses something without being recorded in stereo.
So for me - a voice is a point source and gets ONE mic. Head turns in stereo can sound amazingly weird, but it's an effect, not a sensible technique. If you use two mics, then they'll hear exactly the same thing, and there is a chance that you'll start to get nasty comb filtering because the source is very close to a point.