Decker's claim that his crew beamed down to the planet in order to evade a known planet killer never made an ounce of sense. Either Decker was trying to hide his guilt in deliberate mass murder when sprouting this bullshit story... Or then in his sorry state, he forgot to tell the heroes what the beam-down had been all about.
The Constellation was the most survivable thing in the sector, battle damage or no battle damage; death everywhere else would be much quicker and much more certain. Why abandon her? And why evacuate to a planet soon to be eaten? The obvious answer would be that the Constellation would soon no longer be, whereas the planet would soon be safe. That is, Decker would take down the DDM with one well-aimed starship...
He'd drop that plan once a better one presented itself (in the form of another starship), of course. But he'd return to it soon enough, and by doing so tip off Kirk enough to allow him to complete the job.
Either that, or then Decker was a murderous maniac and/or his entire crew a bunch of suicidees. Which takes us to this, I guess:
An interesting inference, made all the more plausible by the norm of "charitable reading."
The transcript, however, sugests that his reasoning was otherwise:
Oh, I had to beam them down. We were dead. No power, our phasers useless. I stayed behind, the last man. The Captain, the last man aboard the ship. That's what you're supposed to do, isn't it? And then it hit again and the transporter went out. They were down there, and I'm up here.
It seems fairly obvious that Decker was getting his crew off the ship before the Doomsday Machine destroyed it, not reallizing that his ship was no longer a target.
Even if the planet was likely doomed, it still represents a better choice than immediate death aboard a crippled starship which is being savaged.
Is it rational to obey the orders of a gunman, even if you have good reason to believe that he intends to kill you?
Well, if the choice is between die right now (disobey his commands and/or fight) or comply with his wishes and live longer (even though this helps him eventually put you in the position in which he prefers to kill you in) most people would opt to live longer in the hopes of a cosmic deus ex machina.
Even without such hope, if it is preferrable to live an hour longer than just a minute longer, this still represents a rational decision.
Likewise, Decker beaming his crew to the planet (theoretically) was giving them extra minutes or hours for a starship to arrive while Decker went down with the ship.
But Decker isn't actually giving any reasoning, and Spock isn't actually countering any.
Decker's contention is that the first priority of the ship is to rush to the aid of the Rigel colony. His contention is that this is their duty as starfleet officers.
Consider the ficitious battle of Helm's Deep for moment. If Gandalf had not shown up with the cavalry, what would be the responsibilties of the soldiers? Even though the cause is doomed, you still have women and children in the walls fo the keep. Your duty is to protect them to the last, to fight until you are dead. Your duty is not to fight unless you can win, not to run if you are certain to lose, but to serve the people you took an oath to protect.
Spock is merely saying that he doesn't find any merit in Decker's so-called plans, and thereafter ignoring them and him. Decker only gets his voice heard again when he 1) silences Spock with Rules & Regulations and later when he 2) backs down from said plans after demonstrating their ineffectiveness.
It's possible that Decker is completely bonkers. Kirk often appeared to be, too - but very seldom actually was. The crew might have learned to see through that trick already... Although this time to their own peril.
Timo Saloniemi
No, when Decker makes a move that is unambiguously bonkers, Spock threatens to relieve Decker of Command. It is not until Decker makes a move that is directly suicidal that Spock (can) act (and it is apparent that he wishes to). Decker needs to have, at least, plausible grounds for his actions.