A scene paraphrased from Quatermass and the Pit
Quatermass: So, what is this gizmo?
John: It shows us a person's thoughts!
Quatermass: Excellent! We can hook this up to Miss Jones and find out the aliens' plans!
John: It's a neat device! It functions by translating the electrical impulses with the person's--
Quatermass: Just shut up and plug it in!
An online writing group I belong to generally tends to keep on topic, but sometimes it does go off on interesting tangents. Recently, one of the posters took to criticizing M. Night Shyamalan's alien-invasion thriller Signs. It's crime? Bad science.
If aliens were advanced enough to travel the interstellar distances to our planet, how is it that we could defeat them with a glass of water and a baseball bat? Would they care very much about anything we had to offer (resources? Why should they need our resources? They have enough resources to cross between the STARS for heavens' sake!)? Would they be primal enough to want to kill us without having first destroyed themselves in a nuclear holocaust? The critic basically disassembled the movie because these and other logical flaws, which broke down his suspension of disbelief rather like Galloping Gertie in a stiff wind.
My argument was, these elements, while not making sense on a scientific level of discussion, are well-worn science-fiction tools (especially in television science fiction). Most readers and viewers are aware of these clich+s and take them only as part of the plotting vocabulary designed to add spice to a story, or make certain elements of the story possible so that the plotting can begin.
There is a lot of science fiction that plays by scientific rules and regulations (Asimov, Clarke and others), some of it to the point where I think it gets a little dry. For the majority of the genre, however, especially in television science fiction, the "science" part of science fiction is a complete misnomer. Star Trek's transporter is a physical impossibility. The Doctor's TARDIS is less of a time machine and more of a wizard's cabinet designed to transport the lead characters from adventure to adventure. But these are all peripheral issues; the Doctor's TARDIS is the means to get the heroes to the story. Once there, the gizmos aren't important; it's the story that counts.
Despite the scientific implausibility of alien invasion, aliens keep invading. That's because the basic story is just too good to give up: a massive threat from who-knows-where is out to kill us all. How do our heros deal with it? If you overlook the bad science in Signs, what you have left is a tensely directed tale of a single family facing a daunting threat, a house under siege, and a man's faith in God restored. The story could have just as easily been about a frontier family hiding from a ravenous pack of wolves. That would have been more believable, but would it have been Signs?
Science fiction, very often, presents faster-than-light vehicles which are physically impossible. But if they were fictionally impossible, the characters couldn't get out of their solar system. So most readers accept this impossibility and move onto the story. The same goes for Fantasy: magic isn't real, of course, but if magic wasn't fictional, Harry Potter would just be learning his sums while fearing the appearance of mafioso kingpin Don Voldemort.
So, yes, science fiction and fantasy are rooted in the impossible, and science fiction commits acts of hypocrisy almost every day by this very stance. Readers are very forgiving... as long as the writers themselves know that the tools of the genre can only get you so far. Scientific gizmos and magical incantations can't be your get-out-of-jail-free card. The story is about your characters and their conflicts. The toolkit can only take you so far -- once you start telling your story, that's where your struggles really begin.