MT,

When accacompanied by instruments, our ear actually tells us to sing in non-tempered fashion(people run into intonation problems for this reason)--also, though the fretted instruments are fretted, you can de-temper by simply ear tuning to an open chord--another thing about fretted instruments, you intuitively bend the pitches on the frets to alter the pitch to what your ear tells you it should be--Ever played leads on a guitar in closed positions, only to discover that , when you played open chords, you were out of tune?

If you think that this whole business would mean that everyone was in slightly different tune, you would be right--tempering provides standardization, which has not necessarily been there before--in Turkish classical music, the fretted instruments have movable frets, and the players decide for themselves which pitch is the right one for a lot of the scale steps--it works because they don't use chords, and they like the tension that dissonances create--