I was unaware that mainline churches use semper reformanda to defend constant changes to the church so that it fits changes to culture. I can see why they might, and I don't doubt that it's true, and I certainly agree it is a bad way to think... but this wasn't on my radar.
Rather, I've seen semper reformanda used in godly churches to suggest that the church needs constantly to examine its life for sin and to practice repeated repentance—not changes in what we affirm to be true, but constant changes in life because we tend to stray from what is true. That may not be the original intent of the phrase or even a sensible interpretation of it, but it's how I've heard it used. And that principle (always reforming life back toward what is true) seems sound.