It's the only thing that explains it; I just think that TPTB didn't want to leave the transporter room door open without an explanation (generally, the problem of the Scalosians moving through the ship is dodged fairly well). The dialogue in question is: SCOTT: Can we use phasers to cut through that wall, bypass the force field and get at that unit?
SPOCK: Mister Scott, we cannot cope with them on our level.
SCOTT: Can we find some way of coping with them on theirs?
SPOCK: That is a very logical suggestion. Stand by in the transporter room.
And indeed, Scotty's suggestion is very logical (and I like the exchange and how Spock compliments Scotty's logic). But sending Scotty to the transporter room makes absolutely no sense. Even if Spock was planning some sort of forcible beam-off of the Scalosians as I theorized above, he can't cooperate with Scotty across the acceleration barrier and doesn't need him anyway.
I also still can't figure out what Spock's plan was. I suppose he's on his way to life support to destroy the unit, but he should have figured out that his phaser won't (or might not) work against the Scalosians. OTOH, I guess it would still work in terms of destroying the unit, come to think of it. The Scalosians, however, would have at least a few seconds to block the beam with something (based on the operation of Kirk's phaser on the bridge). It seems like a more systemic plan of attack was indicated: perhaps first cutting power to the entire deck? The dialogue is a little murky on whether the "deep freeze unit" powers itself.
