a power plant, in fact the whole power grid, is just like any other engine
you adjust the fuel input to control the power it makes. So energy is not particularly wasted.
If the power being made in the grid is unequal to the power being used in the customers' homes and factories,
then like any other engine the grid will speed up or slow down.
Extremely sensitive speed measurement machinery keeps the whole electric grid at 60 or 50 cycles (Hettz) as Aleph said above.
I worked in a power plant. The frequency recorder drew an almost straight line around 60hz, varying by 0.01 or less. Only one day in 30 years did i see the grid run 59.9 for more than a very few seconds.
Smart Grid is not needed for any real reason that i know.
It's just a big next step in automating things .
Smart Grid is going to greatly simplify "time of use" billing and remote control of every individual wall outlet in the country. Do some reading on "smart grid" products, you'll be amazed.
BTW nuke plants can "throttle:, it's called "Load Follow" but is avoided for two reasons...
1. It's usually more thrifty to throttle some other plant and save its more expensive fuel, burning instead that cheap nuke fuel;
2. Load Following shifts around the heat production among regions in the reactor's core . If it causes unbalance, you may not be able to get back to full power as soon as you would like. Look up "Xenon Oscillation".