I had one day that made me decide to always use leader with braid. I was fishing a pond while on vacation in SC, and the fish were mainly pre spawn feeding pretty heavy. I was throwing a senko on a spinning rod, 10lb braid to 10lb fluoro. Almost every cast was producing a fish, so when I broke off my leader I said heck with it I will go with straight braid, the bite is hot enough that it won't change anything. Went 20 mins without a bite, put a leader back on and continued catching. Never moved my feet.
Just recently in a tournament on Anna, my partner and I were on a solid drop shot bite. I was fishing braid with 8lb *clear* mono as a leader, and he had 8lb "clear/blue" mono with no leader. I was getting tons of bites and he wasn't doing much, as soon as I put a leader on for him he started catching. Leaders definitely make a difference. Even if 9 out of 10 bass will eat a bait without a leader on it, I don't want the braid to be a barrier between me and a bite I could be getting.