Here's a broad topic to ponder. Its got tons of opinions and tons of speculation. Its also a topic that has merit when it comes to fish and how these things work.
I'm not a scientist so mostly all I have is observation and I feel I am pretty good in that department so I'll share what I notice.
Lots of banter goes around about all these various scent used in or on plastic baits but I feel that unless any scent can actually be sensed by a fish's smelling organs its useless. Notice I said "sensed", because fish do not have the sensory devices we have and cannot "smell" as we do.
I think at the top of the pile Berkley scientists as well as some on the Trigger X products side of things are standing tall. I've had more than a great amount of success using all three brands....Trigger X, PowerBait and Gulp. Trigger X is probably more on the bottom of the heap since I generally don't see a need to go much further than Gulp or PowerBait, depending on what I am fishing.
For the most part, baits coming straight out of my molds catch fish without any additions of scent. Sometimes though something more seems to be needed to get hits and what I have observed is that I don't think a cooked-in scent does anything unless the surface tension of the plastic gets breeched OR it is formulated to immediately interact with the fish's smell detectors. Salt may be an exception but I don't deal with any salt/bait relationship in my fishing so I can't comment much on its use. Mr. Skarlis will butt heads on this but I find the 3" and 4" Power Minnows from Berkley as the kingpin of walleye/sauger scented plastics on the Mississippi River. The GULP 1" thru 2 1/2" Minnows are absolutely the devil on crappies and sunfish for me. To a lesser extent, Trigger X catches many fish for me but as a rule I do not care much for bait shapes or styles in the panfish selection. Mepps/MisterTwister years back had an EXUDE product in panfish plastics that was a terror in its own right and I still have packages of minnows for panfish from when I was staffing with the company and they still work as well as the Trigger X products. All of these products are tricked out to be sensed by the fish thru their smelling apparatus. Do they make sense to use? Absolutely. And they work extremely well. At times. At other times the fish are simply aggressive enough to hit baits that have no scent or chemical attractant.
Some independent small plastics companies, especially those in the ice plastic arenas for panfish, have come along with scents that seem to work very well for their customers. What they are using or how they developed them stay a mystery and that's ok. If they work, that's a good thing. I think though that many scents, aside from the chemically engineered scent, are as much a cover scent for masking human - related smells as anything. Outboard fuel, oils, tobacco, foods....all can be a turn off to fish IF they can sense them. With the fish being entire different from us, its really hard to say just how much of US they can react to. So if a fish seems to be less tentative about a bait that has some scent of some sort on it, it makes sense to use the baits.
Panfish and crappies, both in open water and under the ice can be real morons when it comes to color, size and shape/action, but these components almost appear to be forgiving. A quick color shift and a guy can be right back in business. Same with profile or length. Then comes the dead bite and a shift to one of the "super scented baits" and bingo, a guy is right back in the game. I've seen this happen countless times, so again, does it make any sense to use these scents? Absolutely. Anglers can use the knowledge that there are days when these heavy hitting scents just flat out beat a plain plastic to death....as long as the angler also realizes that there are days when sticking to the scented stuff will leave them fishless.
Scents make sense if people pay attention to what is happening and use that power of observation down the road.