Harbinger could've played the Starchild's role just fine, trying to persuade Shepard not to use the Crucible 'cause "our mission is important. the harvest must continue". (regardless of either it "synth VS bio" or "Dark Energy"). He could explain everything. And the Crucible should've had only two options. Control and Destroy without renegade/paragon labels on them. And "destroy" should've been a precision strike OR "control" should've removed free will from Geth and EDI. To be equally acceptable/unacceptable choices.
Starchild was not needed. Its addition wasn't required and had disastrous consequences.