I have no problem with Jar-Jaromir.
Indeed.
I have no problem at all with Jar-Jaromir being in ROTK.
After all, I have heard (I actually have heard this, folks) that Sauron is taking a new incarnation in ROTK.
Perhaps that means he is coming down out of that Great Tower in order to kick someone into the next world.
Methinks that if Jar-Jaromir appears on the scene, Sauron will be coming for him.
Then something shall happen in Arda, that has not happened in Arda since the Forging of the Great Rings of Power.
We will see an alliance and friendship once more between Sauron and elves, Sauron and dwarves, and Sauron with men.
Side by side, Sauron, Saruman, Gandalf, Aragorn, Eomer Galadriel, Elrond, and even Treebeard will take up the noble quest to rid Middle Earth of this most supreme of all threats - one Jar-Jaromir!