There are really several aspects to this.
1) First and foremast - what impact does it have on the fish? I wish I could answer this 100%. I've had success with salted and unsalted baits. Just my opinion - but I would lean towards it not actually improving the fish striking the bait (there's more to this in #2). Examples would be craws and worms (both would float ideally - hence little to no salt in the bait).
2) Salt changes the action - makes a bait harder / sink. This can be a very good thing. In a stick type bait you will normally compensate for this by adding softener. The key here is to recognize, you are not making a bait, you are making a lure. The difference being a lure needs to perform a certain way for ideal results. This means it's not as simple as just busting out something that looks good, but your recipe needs to be tested and tweaked for optimal performance. This is the difference between bait companys and why some baits work better, even though they look identical.
3) Are you looking at this as a fishermen, or a bait manufacturer? As a fisherman I want it to catch fish, and ideally last for a few (however, I would be more than happy to swap plastic for fish all day long). From a bait manufacturer there are other considerations - they want it to produce as well, then there is some variance on how long they want it to last.
4) A bait without salt, silt, bubbles, etc. will be more durable. We can also remelt our old baits. With an additive in your bait you will more than likely go through more baits, add more colorant, softener, and have remelts that absorb water more. I'm not sure how you would totally flush out the cost benefit of adding salt.
End of the day I think it really comes down to confidence and catching fish. If salt helps in either area - do it. To me, this is what I like the best about what we all do. The mental aspect of striving for the unattainable - the perfect fish catching machine.
Tight lines!
Jason