^I don't think that scene in "Spock's Brain" is stupid. In fact, I think it's one of DeForest Kelley's finest moments on the show. The way he delivers that line, "A child could do it!" -- it's awesome.
But there are some cues from "The Enemy Within" that I do tend to associate more with "Arena." And the portion of the "Spock's Brain" score where the Morgs are stalking the landing party and about to attack tends to remind me more of its uses in other episodes like "The Savage Curtain."