It's true it doesn't use much CPU in percentage terms... but wakeups are expensive.
When I'm reading a web page, ideally my CPU is halted, using barely any power. With the screen dim, I might be hoping for 20 hours battery life in that state.
To wake up, just to poll something, and go back to sleep again uses a lot of energy compared to doing nothing. Something polling a few times a second might reduce my 20 hours down to just 10.
