Is any sequel really "necessary"? Yes. Some are. In fact, I would say a lot are. But just as many are not. Take the new Vin Diesel movie Fast & Furious, the third (!) sequel to The Fast and the Furious. I haven't seen it yet, but I can promise you that it is as unnecessary as the last two sequels. I ran down ten of the most unnecessary sequels ever made at MoviesWithoutPity.com, and you may be surprised to see what's up there.
Fast & Furious: Another Unnecessary Sequel