In STID, we see that Section 31 has gained access to the advanced transporter system designed by Prime Scotty, permitting people to be beamed across hundreds of light years. My question is, why doesn't Admiral Marcus simply have Kirk and co beam straight from Earth to Qo'noS instead of going in the Enterprise? Surely by now they've had time to duplicate the technology?
Because that would defeat his whole plan of getting the Enterprise stranded and "helping" start the war he wanted with the Klingons, among other reasons.
Exactly. Just the Klingons finding a group of humans on Kronos fighting each other may not be threat enough (a couple Klingon D-4s could strafe them out of existence), but a starship firing torpedoes to their planet with no apparent reason behind it is exactly what would start the war.
Also, Marcus has a perfect excuse for not using the transporter to send a "police patrol" after Khan. Namely, we have no evidence that the unit could perform the return journey! The device used by Khan was left in the wreckage of his aircraft. How could he activate it for a return journey? How could anybody back on Earth target the heroes and their captive for a beam-back? Even if Kirk took another unit with him when beaming to Klingon space, how could he establish return coordinates? And how could he be certain that Klingons didn't capture that second unit after the heroes beamed out but before the self-destruct charge hopefully fired? Too many problems there for the attempt to be practical. Timo Saloniemi