They absolutely make a difference... sometimes. I agree with earlier thoughts regarding saltwater; most things move way too fast in the ocean for fish to think much about how they smell before deciding whether to try and eat them. Besides that, a lot of the bait we use in the salt is live, so it looks, moves, and smells just like the real thing... cuz it is!
When the bite is on, I don't think scents make much difference. It's that day when everyone's slow that the 10%ers make their hay, and it can be any combination of things (scent being a big one) they use to find success. They don't always seem to know right away which trick will work, but they have a full arsenal of things to try (and keep trying) until they find the magic ingredient. I describe these people in the third person, which tells you I'm not one of them, but if you meet one, they'll talk your ear off about different cures, scents, and other delightful bait accoutrements... fishy folks!