I've always thought that playing open strings was one of the main differences between playing a violin and playing a fiddle. Since most fiddle tunes are almost exclusively in first position using the fourth finger is almost optional. I also agree with the notion of sympathetic vibration that playing open strings gives you. That being said, a lot depends on the melody of the tune being played. Sometimes using the fourth finger instead of an open string makes a passage easier to play. The style you're playing in also makes a difference. You can't get certain sounds or effects using closed fingering exclusively.

radriano