What is so amazing about the voice -- about everyone's voice, whether trained or not -- is its uniqueness. One cannot help but respond to the intrinsically vocal: is the note high for me or low? is it comfortable or not in relation to the other sounds around me? These are the essence of vocalism.
Post-Baroque instrumentalists get that trained out of them. I think that's why so many good ones are going into early music. In Baroque music the voice comes first; instrumentalists are assumed to take their cues from the voice.
I guess that's the basis of my feeling that the instruments and the voices on the harmonia mundi Idomeneo recording are not united in their art. The instrumentalists play as though they understand the harmonies, not the words. I have always found this to be the real difficulty of getting singers and instrumentalists to make music together. In superbly crafted opera or lieder the words reinforce the vocalism of the singer, not the other way around. Instrumentalists, trained to treat all notes as interchangeable, often don't get it.