It's a tricky question, because most of the things you would spend power on in the real world don't need it in Minecraft. UU really is the intended sink for the end-game player.
Light, for example. Once upon a time Notch wanted to make torches burn out and require replacement, glowstone lamps being the more permanent solution. He decided against that idea (and rightfully so - glowstone blocks were much more expensive at the time, though even now, without mods they're a pain to get in large quantities), but you can see how something like that would encourage some rather cool "powered" lighting solutions with mods. Consider having to re-route your power system into any given mineshaft you wanted to revisit 'cause you don't have enough output to light them all at once, for example, and working your way up to lighting a city.
Just about impossible to implement with packs such as FTB though, as there're so many mods that give infinite free light (or infinite free anything, really; BC tried to encourage complex power transmission systems, for eg, then TE comes along with its conduits and subverted nearly the whole system).
The only way I can see limiting systems come into play is via whatever catches Greg's fancy. An well known example of him dealing with this in the past is the Matter Fabricator overriding the relatively cheap "creative mode" Mass Fabricator.
