It depends on where you're fishing.

I usually fish gin clear water for smallies, and I don't get bit much when I use a leader.  A few times, after locating a school of smallies, I've tried using a 30-100 lb wire leader or 100+ lb fluoro leader while my partner's using a 7 lb fluoro leader, and he's consistently outfished me 5:1.  Mostly, I was using swimbaits, so the leader wasn't messing with the lure's action.  I'd really like leaders to work, because I'd avoid pike bite-offs and save tons of money, but they never have.

On the other hand, I've spent a couple of weeks fishing Okeechobee, where I've always tied 65 lb braid direct to my lures, and I've caught tons of fish.

 

I get a kick out of hearing people, who use spinner baits and buzzbaits, saying a wire leader turns fish off.

I use a shark wire leader for my finesse presentations. Great for drop shots and 3/16 ounce jigs.