So I'm watching Alien on cable--for like the 1000th time or some such. (This is the director's cut with Dallas in cocoon, so to speak, so I've only seen this one like 25-50 times.) And for the last 500 times I've seen the movie . . . I've wondered why one would build a self-destruct mechanism in an interplanetary space vehicle in the first place.
If you were really worried about the vessel falling into enemy hands, OK, I can see that. But the Nostromo is commercial. So you would have to be concerned with your cargo falling into enemy (competitor) hands. But the risk of an accidental self-destruct--although minimized by the multiple steps needed to blow the ship up, admittedly--would mitigate against such a step for merely pecuniary reasons.
It's not like the Nostromo was state of the art, or a ship of the line . . . it would be like every Metro bus having a self-destruct system. Or every freight train.
I know, I know . . . it's a plot device.