Pete; the softer the plastic the more realistic the action will be. Also, some freshwater fish can be very picky at times; so the more action the bait has, the better reaction the fish will give.
Softer plastic also has a "hold factor" to it. Meaning, the softer the plastic the longer the fish will hang onto it.
Basically that's the theory.
Saltwater guys want a harder plastic because ; in my opinion; most of the big water fish have teeth and the need for a more durable bait is required.
I'm sure that there is a happy medium somewhere in the hard/soft scale, of plastic, that will allow more flexiblilty while only giving up some of the durability.
Personally I wouldn't care how many baits I went through as long as I was catching fish. So, I would not give up very much softness to the baits I would pour for saltwater.
Just my 2 cents.
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