It
depends on the music and the venue. If possible, using
amplifiers for the guitar and bass is more ideal -- the musicians
will have more control over their sound and not have to rely on the
sound person, and
this is a good thing.
Not if you are the sound guy. One of the more common annoyances was
guitarists with Marshall stack(s) who have more power than the
entire PA
and won't turn down so that people can hear the rest of the band...