I only like to use salt in baits that need to sink,s?nkos , craws, etc. For regular worms and even s?nkos I won't run any salt if I am gonna use them for carolina rigging or dropshots because I want them to have more buoyancy and a slower fall. I don't know if salt is proven to make them hold the bait longer than normal, but that would be a difficult hypothesis to prove or disprove without years of field study. Theoretically, if the salt is going into solution in the water, wouldn't the bass be like a shark following a chum slick in the water?
As for salt on the outside, I have to agree with Seminole Fan that it is a great alternative to worm oil to keep packaged baits from sticking together. You have to think of where that oil actually goes once it hits the water. Do you think it just disappears?