Larger color range, definitely better. Human eyes can see about 12 million variations of color, so the 1 billion iPad Pro color display will be 988 million more than your human eyes can appreciate. How's that for awesome!
I'm pretty sure it was only the iMac that got support for 1 billion colors through 10-bit dithering.
It's important to understand the difference between color range and color resolution. The human eye can see more vibrant reds, greens and blues than any current display can replicate. The human eye cannot discern between every color that even the original 2010 iPad could create. The DCI-P3 color space extends the color gamut (range) that the iPad can display. Your eyes can discern the more vibrant colors, but they can't discern every step
between different colors. Does that make sense?
Here is a sample graph that plots P3 against Adobe RGB and sRGB. The larger area is the range of human vision. P3 expands upon sRGB. Apple likely chose it because it's a bit more balanced than Adobe's green bias. We've still got a long ways to go to completely cover human vision.