I think the major naming problem comes from the fact that nearly all movie sequels are intended solely to capitalize on the success of the first movie. Because of this you have to have something in the name to connect them to the first movie so people will know that they have to go see them.

Movie sequels are rarely produced, written, or directed by the same people, and generally the people who produce the sequels are money-grubbing weasels who don't care about quality and want to milk people's good feelings of the original movie for every cent they can. The truly awful ones are, in fact, barely written or directed at all. They're designed to be as low budget as possible, relying wholly on the strength of the original.

Because of this, you rarely see a third movie if the second one was trash. If you do, it's usually either because it's a kid's movie, or because the good feelings people have towards the original were so strong that someone felt they could get away with more. Notable examples of this are the Land Before Time movies, which have somewhere around eleven in the series at this point, and another comes out every few years, and Superman. The original Superman was highly successful. The sequel had enough strength that even though it was poorer quality, it still generated a decent response. The third and fourth are not Superman movies, and should not be considered such.

Of course, I'm not speaking of the chain series here when I talk about sequels. Such series as the Lord of the Rings, Spiderman, X-Men, etc. were planned to have more than one movie, and each succeeding production was intended to top the one before it in terms of quality.