My grip with that scene is that it further reinforced that you killed one Alien, the movie didn't care. In the first film, defeating the
single Alien was Ripley's achievement, her
triumph over the horror she has passed. This all demonstrated what a
single Alien could do, and what it could inspire with its
presence. It was terrifying. It was... damn
smart. Oh, you thought the Alien was so dumb that he remained on the ship! But no, here he comes
back in the Narcissus, going peekaboo on Ripley, because it
knew shit was going down.
In
Aliens, the theatrical cut, this concept is... a bit diluted and toned down, as a lot of Aliens are either killed or made back away with the pulse rifles. They still can surprise you with their intelligence, they cut the power
of a specific section of Hadley's Hope, and one managed to take down the single chance the Marines had to go away from LV-426. No Monster is invulnerable, yeah, the Mi-Go in
The Whisperer in the Darkness are terrifying even with the fact you can kill them with a shotgun. Because they are unknown. And
smart. 
I didn't mind the changes applied because the film, literally, sucks you in. But in the director's cut of Aliens, the feel, the terror, is completely
chopped down with the sentry guns scene, and it all comes a bit obvious. It felt pointless.
Aliens,
single Aliens, are smarter than that. As said, too, it uses repeated stock footage. I kind of dispreciate when films do that - makes them look like B-movies, which
Aliens is not (it is not a B-movie in the soul, that's what I mean).
Oh and regarding your 'ants' comparison -
but those things ain't ants, man. ;P
