I don't have a lot of faith in the legal system, especially in the US, to hold anyone accountable for violating the terms of a BY-SA license but I think it is a useful shorthand for "I encourage other people to adapt this work, as long as they play along."
The fact that it has a nonprofit and some lawyers behind it is helpful to give it an air of legal legitimacy that carries slightly more weight than me making the same statements without having them reviewed by a lawyer, but I think it is as valuable as a common shorthand than as a legal mechanism.