That's true, you can definitely make it more complicated, but I wouldn't expect a layer change within a layer, I'd expect the whole layer to be changed to the specified height. Understandably, with top/bottom at different layers, this would produce multiple layer height changes within the model, and maybe that would lead to more confusion then its worth.
But, doesn't cura already do some more complicated layer changes with the Adaptive Layers option, although its only applied when certain conditions are met and can't be manually set.