There are usually other clues that the engines are on. I mean, if you saw a 757 cruising along at 35,000ft would you need to look for flashing red lights?
- hey, I got one of those Smiley things without even trying.
But seriously, they're to stop other planes flying into them and to stop baggage handlers from banging their heads on them.
(Sorry, I really, REALLY tried not to.)