I think Rick's right - the key to a good sound setup is the relationship you get going with the sound person. If (as a performer) you introduce yourselves and explain what you're after, and ask the sound person about the foibles of the venue (or the sound gear), then there's more likely to be a feeling that you're working as a team to give the audience the best possible performance.

I love performing without amplification in a venue with great acoustics (sometimes not the venues one would expect - it's always worth doing a check without amplification to see if the venue can wear an acoustic gig). For purely vocal perfgormances it's probably the ideal situation - you can adjust your voice as you hear the harmonies change and make the whole room swim in the sound. That's IF the venue has good acoustics.

If the venue has less that good acoustics, then I really think amplification's the only solution.
If you're a performer, try and get the best sound by communicating with eachother and the sound person, and by doing a proper sound check. If you can, have a tame audient who knows your repertoire to whisper in the sound person's shell-like ear or wave their hands at you in a meaningful fashion if the sound's not working as it should. And if you find a sound person who has a feel for your music, get their name and number (or lock them in the boot of your car).
If you're an audient, maybe gently communicate any perceived shortcomings to the sound person in the first break. It's worth a try.

As Callie will tell you, I do love singing with a mike tho - it's those childhood dreams of pop stardom.

Cheers
- Chicky