[quote=idlehands]
Starscream is the shit, only he could continuously undermine and insult the meglomaniac Megatron and still keep his position and his life. 
[/quote]

Or it sends a stronger message that Megatron is so unintimidated by Starscream's usurper ways that by keeping him alive, it shows he isn't worth Megatron's time or attention, and abusing him like a bad pet in place of execution demeans Starscream's existence. He's probably more amused by his treachery than concerned, by far.

Plus, in pretty much any continuity, none of the Decepticons under Starscream are happy about it and vocally bitch about it, even to his face. It probably burns Thundercracker and Skywarp's asses that Starscream is often their team leader. I think Megatron is hoping one of his minions gets the balls to kill Starscream to be a worthy replacement, because any time a Decepticon does raise arms against Starscream, Megatron finds it amusing and raises no objections.