Personally, I prefer the James Bond approach when it comes to sequels - a unique name for each film. Imagine if the producers had gone the other route, and Dr. No had been called James Bond, followed by James Bond II, III, etc. James Bond XXII doesn't look nearly as good on a marquee as Quantum of Solace. Everyone understands that Chinatown and The Two Jakes are connected narratives; how lame would Chinatown II have been? It sounds like a new subdivision.
Sometimes a numbered sequel makes sense. Not only was Quatermass 2 the second film in the series, it's also the name of a rocket in the film, Quatermass' second ship. Robocop 2 employed a similar conceit.
But French Connection II is, well, just a convenient, marketing-friendly name for the sequel to a popular film. This is how it goes with a lot of sequels - tack a number after a brand, and bam, you're good to go. Sometimes it makes sense, but other times you wind up with abominations such as 2 Fast 2 Furious or TR2N. (How do you even say that? "Tr-too-n?"?)
Take the Superman films. Superman was followed with II, III and IV, subtitled The Quest for Peace. Superman is at least as recognizable as James Bond - there's no excuse for lazy titles like these.
Superman II featured three Kryptonian bad guys who survived the explosion of Krypton thanks to Superman's dad, who had thrown them into the Phantom Zone (read: "space jail") before the planet blew up. So in a sense, Jor-El was a father-figure not just to Superman, but to the three villains; he gave them deliverance from doom, if accidentally.
The three villains dressed in black for the entire film, and they had all of Superman's amazing powers. However, being villains, they used their powers for evil. Superman II could have been called, say, Dark Legacy, and marketed thusly, aping the Bond style:
"He saved his only son from a doomed planet..."
"But Earth's greatest champion wasn't Jor-El's only legacy..."
"Christopher Reeve is Superman - the Man of Steel - in Richard Donner's..."
In French Connection II, Popeye Doyle, the protaganist of the first film, continues his obsessive pursuit of an elusive French drug lord. Doyle steamrollers over all opposition to pursue his goal, but on the way he winds up getting hooked on heroin himself. Since the film is about obsession, and the drug comes in the form of white powder, the film could have been called White Whale, with a nice double meaning.
In Superman III, Clark returns to Smallville, rekindles an old romance, and faces (of course) kryptonite. If the producers could have seen the future, they could have stolen a great episode title from Lois and Clark: The New Adventures of Superman - "The Green, Green Glow of Home."
What about films that take their titles from TV shows? Star Trek is the first example that comes to mind, and the first film in its big-screen series was called Star Trek: The Motion Picture.
That is a terrible name for a movie - at least from an artistic perspective. The original story was called "In Thy Image," an evocative title that would have been a far better fit for the film's ponderous dignity. At the very least, they should have just called it Star Trek - we know it's a motion picture, there's no need to hit us over the head with the information. At least the new movie is going that route. (But then, what choice did they have? Were they going to call it Star Trek: The New Motion Picture?)
Sequel or not, a film's title is an integral part of the work, as important to the feel of the piece as any other facet of the art. I wish more producers would recognize that.