AT THIS TIME.
That's precisely the point, Captain. We don't
know it'll do. Moving into that Cloud,
at this time, is an unwarranted gamble.
It means Decker didn't think they should do it at the moment Spock recommend they proceed. They had just been attacked, and while the second attack had been broken off after Spock's sped-up message, Kirk had not even attempted further communication. Maybe as Captain, Decker would have tried that, and gone in after not receiving further reply. I mean, what else could he do? Follow the cloud and keep saying "Hello?" At some point he'd have been forced to go in. Kirk was just bolder, taking "they're not shooting" as an invitation to proceed.