Lately I have noticed that many pilots at Palo Alto Airport (PAO) have been reading back nearly every word of ATC transmissions. This differs from the way I was taught, and seems like it could lead to unnecessary frequency congestion. (PAO is a very busy GA airport, especially on weekends.) It also seems like it could distract the pilot from flying the airplane, looking for traffic, etc., especially in single pilot operations. However, I'm wondering what others think of this practice. How much do you read back and why? Does your readback practice differ depending on whether you're flying IFR or VFR?