To be accurate, the P2.5 *DOES* show the current voltage level, under load. It is as good as or better than a remaining capacity icon or percentage. Keep clicking the fire button until "CB" displays. It then fires the mod and displays the voltage level.
ETA: Not to suggest that carrying a couple spares is a bad idea. I always carry spare batteries, regardless of the mod. And with a mini running 18350 cells, I'd carry 4 but I am a Chain Vaper