Doctor Who is not just about the Time Lord and his companion. Each new episode has the opportunity to introduce new, potentially one-off, characters to flesh out the story. Although they may not be the stars, and may never even be seen again, these characters are often what will define the episode in the long term. You can have the Doctor meet historical figures like Shakespeare, Dickens, Churchill or Van Gogh. Well, not those examples, because theyve already been done. But you get my point; cast a good actor to bring such a character to life and allow them to form an entertaining relationship with the Doctor. Alternatively, you can introduce characters simply to allow for further interaction with the Doctor. Im particularly thinking about James Cordens Craig, or the Paternoster Gang (Vastra, Strax and Jenny). In these examples, the characters worked so well, they went on to appear again. I suppose Captain Jack Harkness fits this bill too: he showed up as a supporting character in The Empty Child and went on to get his own series. Wow. So yes, its important to have characters that people will like and can add more to the episode than simple plot points. But of course, if you like.