I remember Kirk assuming there will be another assassination after hearing of more peace talks, but I don't remember a line about Kirk and McCoy being the only ones who could work the case.
Kirk says to McCoy, “You and I are nothing. You heard the judge. The peace conference is on again. Whoever killed Gorkon is bound to attempt another assassination. Unless we can get out of here.” That’s the only explanation ever given for why, in Marta’s words to Kirk and McCoy, “Somebody up there wants you out of the way.”
It’s just brilliant. The Klingons are afraid of Kirk and McCoy escaping Rura Penthe and thwarting the second assassination. They decide the best way to prevent them escaping is to have Marta first verify that they have a way off the surface and then help them get out of the mines and out of the beaming shield so that the Klingons can use the escape attempt as an excuse to kill them. But, surprise surprise, Kirk and McCoy beam up to the Enterprise! D’oh!
The whole movie is rife with this kind of idiocy.
I wouldn't toss the movie for having one dumb line. It already had one rather foolish scene involving the Klingon border patrol and a bunch of dusty books...
Neither would I. In fact, it has a lot more than one dumb line and one rather foolish scene, and I still wouldn’t toss it. It’s a very fun movie, but it’s also a very stupid very fun movie. It’s enjoyable if you either embrace the stupidity or shut down the thinking portions of your brain so that you don’t notice the stupidity.