Even if you don't care about OLED's superior contrast, there are benefits:
- Significant battery life improvement (often double) when using apps in Dark Mode.
- Smaller dimension -- OLED is thinner and requires no side bezels.
- Better motion handling. More natural looking motion, with less blur and faster response.
- More uniform performance across the entire screen. No darker patches, etc.
- Wider viewing angle. Doesn't matter too much over small screen of course.
And as for "always on display", while iOS does not yet utilize it, things may change, perhaps even as a surprise new feature on iOS 12 and new iPhone.