Well, the crusher is basically a praetorian on all fours.
The reason for the new aliens, the crusher and the spitter so far, is to vary game play.
If you are fighting the same aliens every level, and every match, it gets boring.
Think about Left 4 Dead. If the only infected you ever fought were the horde, or one of the special infected, it would get boring.
Same with Halo (Jackals, long range, Grunts, close range minor, Elites, close range medium, Hunters, close range major) or Team Fortress 2, or even something as simple as Mario. Their are different hostile NPCs to make gameplay more dynamic and fun.
I don't think they are for merchandising, or are ever going to show up in a movie.
I know it got on my nerves in AVP 2010 that the aliens didn't have any ranged attack. If Marines have a corner locked down, and you are the Alien, nothing you can do really. So the spitter can help even things out.