Yeah they're Ranger class elites. You get a medal on the good ol B.net if you kill them. but they're very strong and yes, will dissapear. So you gotta kill them fast.
I have noticed alot of very high brow inconsistencies with canon. Mainly the very instance that Spartan-IIIs were on Reach and not Spartan-IIs who were really the ones fighting the invasion force. Also the Pillar of Autumn was never on the surface of Reach. As for your problem. They may have...
I never really got any good lines out of marines in Reach. Halo 3 had the best. There mine would be when a brute yells "We don't accept American Express!" But in Reach the firefight voice of sergeant Stacker has some great lines. My favorite is a rarer melee line "Well theres one dead bonkedybonk."