When it comes to salt impregnation I think it matters most in ROF(rate of fall) and the soft plastics' texture on the outside.

The ROF is important because with more salt in the bait the faster it will sink. This can be crucial to how your bait is presented to the fish and can dictate if that fish will bite depending upon it's mood. What I mean is if the fish are shallow and in a nutrual mood that fish might need to see that bait for a longer period of time before it is enticed to strike it. In order for the fish to see the bait longer the soft plastic must have less salt so it is less dense therefore causing a slower ROF.

Another sinerio would be fishing deeper or more agressive shallow fish. If the fish are more agressive that faster ROF  would cause the fish to make the choice of striking the bait faster. Also when fishing deeper that extra salt in the bait would make it more dense allowing it to get down to the fish with out wasting time in "dead water".

The salt will also make a difference in the way it looks. Most things in nature have a rough outer edge. Look at a crawfish. It has a rough outer shell. When you but tubes a #1 crawfish look alike bait alot of them come out of the package with a smooth shiny surface. Yes they may have salt on the outside that washes off on the first cast but Get Bit tubes have that good rough aperance from all the salt that is contained in the plastic. That salt gives the tube the the natural look that fish are used to and bite good and hard.

A last reason why salt is important to anglers is the scent factor. I personally use a comercial scent but that is not to say that some people have more confidence knowing there bait is scent possitive.