"If they added battery packs which the Nixie could recharge from to compensate then it kind of defeats the point of it being wearable and so portable."

Would depend on how it's done, of course, but just because something is able to use battery packs doesn't force me to carry them with me all the time. Just having some I could keep in a car to replace quickly while 'out' would be sufficient for some (many?) use cases.