...with only Bob and Justin around, Harry really did need a solid, positive influence in his life.

That was my reasoning too. McCoy brings that sense of family in the books, too - someone who is looking out for Harry when he seems alone a lot of the time. Harry has Bob in the tv!verse, but, as you said, he needs someone who can keep him on the straight and narrow.

Thanks for pointing out the mistake, I'll fix that up. :-)