They throw them around like marketing terms because they are all about marketing and to an additional extent, product development. Tube MFGRs develop new products to compete with each other. And they market them so that consumers will demand bikes made with them and thus drive sales. Because, as we all know, newer is always better.

Builders have a vested stake since a given tubeset is easier/harder/better/worse to work with and because they can be translated into a faster/heavier/slower/lighter/compliant/stiff frame depending on the consumer's particular needs. But by and large, big bike MFGRs offer frames in new tubesets in order to compete with their rivals and to attract our dollars. Small custom builders use certain sets because they can be built in a way they want their products built.

For the bulk of us though, it's really unimportant unless we have specific needs. If you want a 2.8 lb. steel frame, 531 is probably not going to cut it but S3 might. If you want a robust dirt road touring bike, S3 or Ultra Foco are probably not good choices. If you want a lugged steel retro model, S3 again is not going to be the right thing. But if you're just looking for a steel frame of a reasonable weight to ride the roads, just about anything can be built the way you want it.

Re: stainless. It's nothing new. The current offerings are just easier to work with than the old offerings and the supply side appears more robust. There were problems with earlier versions of stainless and thus they never caught on among the builders. There are also problems this time around and most of the custom builders I have worked with have told me they expect the product to fail dramatically in the marketplace. However, there are always a few nutcase steel fans who will have to have one, just because, and so there will be some sales via those willing to put up with the pain of working with the stuff. I fall into that category which is why I've been riding a 953 frame since last October.