It's a damned if you do and damned if you don't kind of deal.
In terms of books, how many times have you heard people complain about the Harry Potter films cutting too much out of the book to make a movie. Then they split the last book, and people complain that it's just a cash grab. Sure, it's a cash grab but if you want that extra material from the book who are you to complain?
You don't mind watching 40 hours of Game of Thrones but heaven forbid you get a 6 hour movie split in two.
The Three Musketeers from the 70s was the first to split one movie into two. Kill Bill did it. Now it's the new thing. It's neither good nor bad. It should be taken as a case by case scenario.
I tend to agree with this statement. With some things, it's needed, like Harry Potter & The Deathly Hollows. Other times, it's just a cash grab, like with Peter Jackson's Hobbit Trilogy. It's all strictly a case by case thing.
With massive book adaptations, it's certainly understandable. Books like The Lord Of The Rings (I know it's sold as a book trilogy, but it was written as one long book), or several biblical stories would require this type of film making/story telling.
Take Stephen King's IT, for example. It has been languishing in Development Hell for years now. The made for TV miniseries is a total of 3 hours without commercials (4 hours with). By splitting the story into two, they were able to get more from the book into the movie. Now their plan for a theatrical remake is to do the same thing, but with a larger budget, and not having to worry about an allotted amount of TV time, they can make each half 2.5, or even 3.5 hours and fit far more of the original story into the film(s). I never appreciated until after I had read the novel last summer JUST how much had been cut in order to fill the allotted amount of time.
With The Hobbit, they're taking a 250 page children's book and stretching it out into a three part trilogy, with each part being 3 hours long, just because they did it with The Lord Of The Rings. With The Lord Of The Rings, this was justified. With The Hobbit, it's just a cash grab.
With works that aren't direct adaptations of a specific book or graphic novel, then you're getting into a gray area. Even if they're adapting characters from a comic series (Avengers, Justice League, Guardians Of The Galaxy, etc), if they're not adapting a specific story (Death Of Superman for example) then they're just planning on a series of sequels, and not necessarily splitting one story into two. I wrote a teaser trailer for my Thundarr movie's post credits. That doesn't mean the story of Thundarr's origins is split in two. Only that I hope the movie will be a success and people will want to see Thundarr, Ariel, and Ookla kick some more ass.
Then you have original works like Kill Bill, that aren't derived from any source material. In this case, it could simply be a cash grab. Or the story just wound up being too epic to be told in one movie. That's what happened with Star Wars. George Lucas just found his Space Opera was far too epic to be made as one film, so he took the first act (which was about 100 pages), tagged on the last few pages of the final act (the destruction of The Death Star) and said "Here's your movie". Then he reworked Act 2 and Act 3 to be the other two thirds of a film trilogy, assuming the first one was a success.
In any case, you can't paint them all with the same brush. You have to take it on a case by case basis. If you start refusing to see films because they're split into two or three, you could wind up missing out on some really great movies.