actually narratively it would have been fun if in tf armada starscream was the only one who could activate the star saber because hes the only one who wins their trust. like jetstorm, sonar, and runway respond to oppy because he has the matrix but they form a sort of "faux" star saber when combining for the same reason---optimus is still a leader of a cybertronian faction and therefore the possibility exists that he might put the needs of the many over the wishes of the few (the star saber minicons in particular). So the false saber is a failsafe in case he isn't as genuine or well-intentioned as they believe (maybe it's less powerful overall, and any one of them can easily eject themselves to release the others if they felt they were being used). Similarly they respond to megatron because he forces them to combine (using his special electric abilties), but again they don't combine into the true star saber. They are effectively trapped by megatron in weapon form until he decides to release them, so they resist the possibility of combining into the real thing at the risk of being corrupted (i.e. becoming the dark star saber).
Starscream, by virtue of taking care of the minicons and actually seeing the minicons as their own selves, due to being isolated among his own group, and not having as deep a stake in the conflict as optimus, gains the trust of jetstorm, sonar, and runway. He doesn't start out as the nicest bot to them and still sometimes gets short-tempered with their antics, but the three can see that he's actively trying to be better for their sake. And he's the only jet around, which automatically creates a stronger bond between them. So when starscream needs to defend himself from megatron, the trio finally combine to form the true star saber---i'd also like to think star never returns to the decepticons, because his love for the minicons outweighs any remaining loyalty he has to the group, and returning to megatron with the star saber would break the trust he has with his little guys.