[Man, this movie meme is going to be difficult. With TV shows it was easier, there was a more condensed selection and I could come up with something sorta new nearly every other time. I'm gonna have to actually use my brain to suss this one out. Bollocks.]
I. The DVD market
The very definition of movies that don't need a sequel but get one anyway is the direct-to-DVD market. Universal, Nu Image Films, National Lampoon, Disney, and SyFy Channel Original Movies are the worst offenders of them all.
Universal Titles: Road Trip 2: Beer Pong, American Pie presents, The Scorpion King 2, Dr. Dolittle 3, Dr. Dolittle 4,
Disney Titles: Bambi II, Cinderella II, Cinderella III, The Little Mermaid II, The Little Mermaid III, Inspector Gadget 2, 101 Dalmations II, The Lion King 1/2, The Lion King II, Mulan II, Tarzan II, Pocahontas II, The Fox and the Hound 2, Brother Bear 2, George of the Jungle 2, The Hunchback of Notre Dam II, Air Bud 4, Air Bud 5, Air Bud 6
Nu Image Films: Shark Attack 2, Shark Attack 3: Megalodon, Crocodile 2, Cyborg Cop 2, Cyborg Cop 3, Cyborg Cop 4, Octopus 2, Spiders 2
SyFy Channel Original Movies/Sony Entertainment: Lake Placid 2, Lake Placid 3, Anacondas 3, Cube II
Granted, it could be argued for all the horror film sequels that there is a market for these titles, and that no matter what, they sell, and they're loved. Same goes for the many, many, many Disney sequels that land in the direct-to-DVD market. Point is, they shouldn't exist, and they have no reason to exist - at least not in a creative sense. They're purely there for profit, and the product themselves are utter crap [and yes, this is coming from much experience with the Nu Image, Syfy, and Universal titles]. If a direct-to-DVD sequel will be made, then all I ask with that limited budget is that you have a passable script, and by that, I don't mean one that hits the 90 minute quota for television airings. None of these sequels should have been made, and I'm simply grateful Universal hasn't chummed out the Grease 3 and Deep Blue Sea 2 sequel ideas they've tossed around since late '08.
II. Theatrical Films
A lot of people point out The Matrix and Pirates of the Caribbean sequels as a answer for this question, and I'm not one of those who think they're needless and/or suck. For both franchises, I love the second installment out of the trilogy, and I am very, very thankful they exist and I can watch them for years to come. So, honestly, I find the answer to this topic rather difficult. Now for 1996's Joel Schumacher Batman & Robin, yes by all means that was a horrible, horrible movie, but in retrospect, it sorta needed to happen for Batman Begins to come out of it. We needed the worst possible film to bring the best possible film. Do I wish it could have been avoided? Abso-freakin-lutely.
So, I'm content with saying Legally Blond 2: Red, White and Blonde. That was a needless, simply bad movie no matter what way you look at it. Not even the adorableness of Reese Witherspoon could save it.
And in the category of sequels that I wanted but ended up sucking, I present The Pink Panther 2, the Steve Martin one from 2009. Loved the first film, but the second was just atrocious. OK, semi-rant done.