Not being a metalergist I don't have much of an opinion except for an understanding that stainless tends to be expensive and hard to work with. But, it obviously works well for kitchen sinks and pots and pans...

I went to the Salsa site where there was lengthy writeup on the Vaya and its history. Much of that history evolved around packing it for travel and when they got to the stainless Vaya Travel they said:

"From there, we add S&S Machine stainless steel couplers to allow it to fit into an airline-legal case, we add Alternator dropouts for the versatility and ease of packing mentioned above, and we build it using quadruple butted stainless steel. We chose stainless as a result of discussions about durability both inside the case, and in use in the varying environments of planet Earth. Stainless is very strong, reasonably light, non-corrosive, and you can easily buff out any scratches as a result of case travel. The fork was also changed to have an IS disc brake mount for ease of caliper removal and re-installation, and we chose a simple decal for the graphics, so that it can also be simply replaced if it is damaged during transport."

From that I take it that the stainless is mostly there for cosmetic reasons to make it easier to repair scratches and the like.