The problem with showing a Klingon ship attacking the Enterprise early on on “Errand Of Mercy” will leave the viewer wondering why no other Klingon ships would be shown when a Klingon squadron or fleet shows up later over Organia. Remember the D7 hasn’t been designed or even conceived of yet. Mind you we don’t get to see a Federation fleet either when Sulu returns with reinforcements. Mind you not a truly big deal, but there it is. Even with a bit more time and money it would be impossible for them to design and build multiple ships for a story and villain that might never be seen again.