Solubility of elements is what affects the life of the membrane and how it can scale. Take a piece of marble and try to dissolve it in water. Do the same with salt. You will notice that the salt is highly soluble. It is this solubility that prevents it from scaling a membrane. It is much more complex than I described, but you get the point. http://www.lenntech.com/antiscalants.htm Here is an interesting sales pitch for anti scalants. Softening is not an economical method for treating larger RO systems. Smaller units are fine. For residential units, the membranes are considered diposable because the cost of treatment can greatly exceed the cost of the membranes.
As to the TDS of softenerd water, since it is an Ion Exchenge process, Softened water will usually have a higher TDS than unsoftened water and the handheald TDS meters are not designed for serious analytical testing, they are for information, comparisons, etc. It is what makes up the TDS that is the important factor. The sodium is much easier to deal with than the calcium and magnesium in the water since has nearly no scaling potential.