No doubt Alien has its share of stupid character moments. But they had their reason. And reasons that were used well enough too cover the stupidity of their actions. They weren't the characters in Halloween or Freddy that I feel were stupid (lets pass out on the front porch with a madman stalking the streets). I did root for Freddy/Jason/Meyers in that series, but not so much the original film.
That said, Cameron ALSO gave his characters a reason to cover their stupid actions. Overreliance on tech, and overconfidence in that tech to be the primary reasons. Yet I don't hear many people calling actions like leaving your only link (dropship) to the Sulaco undefended "stupid" when your comrades deaths are being broadcast over air, or the fact Ripley went back to the colony with the Company stupid, or the fact that Apone is trying to follow the orders of an incompetent commander so intently he's dropped by an Alien (ask my WWII vet Grandfather how that REALLY would've went down:f**k him, we are leaving), or the fact that despite listening to both Ripley and her disc, the characters still unleash loads of ammunition at targets still near friendlies knowing the creatures were full of acid.
If you look past the characters logic, and boil down how the plot works, of course its going to look formulatic. But thats the thing with writing and film, you cover that up with the actions of the characters, give them motives as to why they act that way, etc etc etc. Simply adding for the effect of fear on the human psyche can cover up alot of bullshit actions by the characters. How many completely lucid people do you see walk away from a car wreck? As long as the reasons for their actions can be explained with any sort of validity, I feel the "idiot plot" tag is a little out there.
The overreliance on tech is a theme directly carried over from the first film. The only two characters with a military history left after Kane burst are subsequently the only two people that attack the Alien head on until Ripley finishes it off in Alien. As a writer, you don't think the fact that they were former military had anything to do with their mindset? It helps explain the rationale behind Dallas and Parker's motives don't you think?
And there are plenty of times in Aliens the, "You don't go anywhere alone," incident happens as well. Ferro and Spunkmeyer in the dropship. Ripley's initial forays into the vents, Burke's trying to escape from the marines and Aliens (as well as his sabotage with the facehuggers), etc etc etc.
Since both films are identified as being cross genre (and I'm not pulling that out of my ass, check the critic reviews on imdb, or the page for the movies themselves), you can't judge them souly as science fiction/horror films.
They all have elements of thrillers, fantasy, drama, comedy (in Alien Res's case) as well as horror and science fiction.
I would actually go so far as to say that the Alien series is actually "hard" science fantasy. Since bad technology (evidenced in every movie INCLUDING Aliens) takes a backseat to human/creature interaction and interpersonal conflicts.
Of course, I wouldn't think it, but this is looking at it from a film students perspective, maybe we look at it differently since we see written stories translated on film. Our terminology might be different from that of the stories that are told on paper.