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"A wizard is never late, nor is he early. He arrives precisely when he means to."
The firing of the rings in Halo 3 is done after Halo's construction is finished, thus preventing the cannon from blowing the Ark and Halo to pieces and ultimately preventing 343 Guilty Spark from turning hostile and killing Johnson. Two lives would have been saved (however there's a chance gravemind would still try rebuilding itself on Halo so this would eventually and quite possibly lead to the inevitable destruction of installation 04, meaning Guilty Spark would still descend completely into rampancy).
[Edited on 07.02.2012 2:20 PM PDT]