About 50% of the flying machines you are likely to run into on a Saturday in July have a transponder. I am an ex glider pilot who thought his lookout was very good. My aircraft has a bubble canopy which gives very good vis. My PCAS is about twice as good at spotting traffic as I am. Most of the aircraft without transponders are at the slower end of the spectrum, so you are more likely to see them. The turbo prop which hits you up the back doing 160kn will probably have a transponder.
See the summer edition of flyer if you want more.
Rod1