Deathdrop,
Human,
12 years ago
The tail chokes you, thus forcing your mouth open. If you vomit, you'll be choking to death on it, because that thing IS NOT coming off. Period. It's not an effective monster if there are easy ways to prevent it killing you.
Again, I'm going to come back to narrative convenience: A facehugger has to be a death sentence. The second it isn't, the movie stops being scary. If clamping your teeth together worked, Kane would have done it, the Alien never would have been born, and the movie would have ended right there.
There's a reason 90% of the facehugger-related dialog in these movies consists of some variation of "Dude, we seriously can't get this off."