I definitely prefer salted butter because I am a big believer in seasoning. And for me, it adds a subtle layer of flavoring in the production of a recipe. I find that under seasoned food is quite common, even among some professionals... But I also think what the butter is being used for may also determine one's choice of salted or unsalted. For instance, I use unsalted butter when I am clarifying, etc... Although some claim that unsalted butter has a more delicate and fresh flavor and they also see the virtue of unsalted butter in the reduction of sodium intake, for me, I don't really think, in the final analysis, that it makes a significant difference. It's my preference to use salted, but in fact, they can be used interchangeably, as I often do since I generally have both on hand at any given time...
Butter: Salted vs. Unsalted? - Page 2
Well, no more than adding salt directly to the recipe, no?
Given the amount of salt, I doubt that it would have any significant effect on the shelf life of pastries. I mean let's get real here.
As a preservative, salt is used in rather large quantities. Large enough, at any rate, that any foodstuffs preserved that way taste distinctly of it. I have, in fact, been biting my tongue about salt butter, nowadays, having a longer shelf-life than unsalted. I suggest that if anyone actually perform a test they'd find no significant difference, no matter what use-by dates say.
Salt butter (in the U.S. at least) is offered because many people prefer its taste. That's pretty much the only reason for it. Manufacturers work on the conclusive implication that it will be refrigerated, so other preservation methods are unnecessary (as well as adding cost).