Another classic bloviation fest.
I can tell you why progressive-rate springs are often recommended by vendors: If the vendor can convince the customer that it doesn't matter if the customer weighs 140 or 340 lb., that one progressive spring can be shimmed to work perfectly, the vendor only has to stock one spring. If the customer is foolish enough to believe this, and afterward is satisfied and even testifies to this technique's effectiveness afterward, then the customer lucked out and happened to weigh in the range that the spring is appropriate for, or he just fell victim to the power of suggestion.
I don't even try to be so bold as to make such a silly argument (that one progressive set can work for all customers). I carry progressive springs for many of the Guzzis, but they are not all the same spring rates. If a guy has a T3 but is heavy, I sell him the heavier springs for a Cal II, for example. Vice versa, if he is light. For the really sporting guys who have bikes for which one can order straight-rate springs, I try to convince them that this is the best way to go, and it is the best way to go, provided you can help him choose the right springs. This takes experience, and sometimes we have to exchange for a lighter or heavier spring after he's tried the one we thought would be perfect.