I rarely use the pick-up; never when there are no drums or electric instruments. Even at festivals that cater to acoustic music, the sound men will often seem annoyed when they find out that I want to play through a mic. I like to be able to move close to the mic for solos and back off for rhythm.
I recall the first time I ever heard Doc Watson play with a pick-up. It was awful. He might as well play an electric guitar. I've heard him since and the sound has improved considerably. Either he's tweaked the EQ or is using a better pick-up.
A couple of years ago, I saw Steve Earl, John Sebastion , Country Joe MacDonald and some other folks play at Hamilton, Ontario's Festival Of Friends. Country Joe was the only one who played into a mic and his sound was mech better than any of the other musicians who played that day. Often the pick-ups tend to sound twangy and quacky.