I really got lost looking and reading through this thread.
If you have everything sync'd to the US SWOP defaults you wouldn't notice the change because by default the Output Preview opens with your Acrobat's Working Space as the Simulation Profile, so there would be no value change.
This might not be a problem in practice because by default the print destination is the same as the Working Space when a profile is not included.
The conventional wisdom has been if you don't include a CMYK profile the CMYK values should output unchanged—as with Press Quality or High Quality Print presets. It doesn't look like that's the case anymore.
So it seems like at least including an Output Intent (PDF/X-1a) is more important now.