Graphic designer, photographer, UI/UX app designer here. White is white. A good calibration will get it as close to pure if possible. If anything slightly cool is better. Back in college in color photography we'd calibrate everything to 6500k daylight lamps and use color filters by hand to adjust point values for tweaking our enlargers. In digital we'd use whatever version of Spyder was out at the time to get it as close to pure white as possible because we're matching to Epson printers which print on nearly pure white RR paper, and that was also calibrated using a device that scans the printout. Then if you wanted to go for your certain "look" in Lightroom, you could balance warm or cool easily and get accurate results when printing.
As for the purpose of this new feature, it's only supposed to warm the color balance for nighttime. It's unfortunate for those who have held out hope for a full color balance control mechanism. It would be great if we could install system-wide color profiles for our iOS devices. If I'm remembering correctly, at present, you can only change ICC profiles inside an individual app. Apple has come a long way over the years in being more consistent with the manufacturing and color balancing of their devices. But they're still not perfect, defects happen, color shifts over time, etc. To take the iPad Pro seriously as a design tool, it will need the capability for system-wide color calibration (along with many other things).
