Power usage..... you'd be waisting energy just sitting idle
No when it is sitting there with no button being pressed it does not waste energy because the impedance of the input is so very high. Therefore the current flowing is so small the actual resistor size is insignificant.
It is only the small period when the button is being pressed is energy being used. It is not wasted because it is ensuring against interference. So you can reduce the energy at the expense of being more susceptible to interference. Like most things in electronics it is a balance.