Character mistake: Elaan says "If I have to stay here for ten light-years..." A light year is a measure of distance. (00:24:03)
Character mistake: Watkins is asked to check the bypass valve, there the woman appears, asked about the panel, it's obvious she's not part of the crew. Instead of calling security or Mr Scott he starts to describe the panel, which I'm assuming would be classified information.
Character mistake: Spock tells Zarabeth he came from a world "millions of light-years away." The galaxy is only about 100,000 light-years across - the area the Federation has explored even smaller.
Chosen answer: If they're in orbit, they're being pulled along by the planet's gravity well, therefore, impulse engines would only be used for minor corrections and would be "on standby" while in orbit, but not active. (Like keeping your car idling without revving the engine and creating plumes of exhaust).
Captain Defenestrator
Thank you for the info.
Movie Nut