While you're correct in saying one has to introduce characters only if they make sense, I disagree with the notion that J. Jonah Jameson is the ONLY character that has to be introduced.
First of all, we know that Norman Osborn is going to show up in these new movies at some point, given how Oscorp appears to be at the center of virtually everything. And if Norman is there, then Harry also has to be there, as Norman's relationship with his son is a key component to his character, not to mention Harry becoming Peter's best friend is also vital for what is to come. Also, let's not forget that it's more than likely that, at the end of this film, Captain Stacy is going to die, and that Peter and Gwen break up because of it. Well guess who would walk into Peter's life to run interference between them ever more? Mary Jane Watson, of course. Remember, in the original comics, Aunt May arranges for Peter to go on a blind date with her, and how great would that be to have a movie version of "Face it Tiger--you just hit the jackpot" scene? Plus the dynamic between Peter, Gwen, and MJ is considered to be the classic love triangle in Spider-Man comics.
