I have two ideas about the Dregs: 1- Young Fallen who didn't still fully grow the additional two arms, the two growing limbs are covered to not let them receive any kind of damage that would alter their growth. 2- Fallen have a caste based upon genetic supremacy. The Fallen that are "a step behind" in the evolution are the Dregs, which can't fully grow the other two arms (which they keep covered for shame or purely for protection). At the top of the chain we find captains and "bosses" like Rixis, Archon Slayer (Obviously we don't know whether they are just "war beasts" or they actually have a rank) which have 4 arms and are physically more powerful than the other sub-species. Also, I hope Bungie will give us the real names of the species like they did in Halo eheh (Vex can stay as they are, it sounds cool and alien-ish)

I have a feeling dregs are like the grunts of reach. They aren't as physically powerful enemies and often use weak weapons. That being said, high numbers could cause problems... I don't think the dregs arms were amputated, but that rather they were bred to be lesser than the other classes of fallen, demonstrated by their number of arms.

I think it's based on a caste system. There are various houses of Fallen, as has been stated. It could be that the lowest of the caste system has their second pair of arms removed as an emphasis on how "worthless" they are compared to the ruling houses. It also wouldn't be surprising if the Dregs themselves were not apart of houses, but merely shared labor and the such. Slaves and objects of trade in the end. Dregs could also be from houses that failed to survive, were wiped out by enemies in past conflict, or tried to separate from the fleet, and as a sign of their disgrace to the Standing Houses, are striped of their secondary arms. Children among the Dregs would suffer the same fate, fathers sins passing to his sons and all. EDIT: If you become sympathetic to the Dreg after reading this, please replace the above text in your mind with this. The Dreg are all the fallen who've ever become so angry at the enemy that they forcefully ripped their own arms off and beat to death their foes with their dismembered limbs. Hence the caps. The caps are the stubs that remains from this vicious event. And if you can listen closely past the dregs vicious growls and snarls you'll hear their stubbies angrily bouncing around, wanting nothing more than to touch you and gross you out while the other two arms stab and shoot you.

I always thought of it this way: The dregs are part of the main stream fallen race. And like we have people with different traits, such as having more than twenty extremities, so do they. I figure the number of arms is controlled by recessive genes. And like many societies on Earth do today, the fallen abuse the dregs and spend all their resources preparing the (supposedly) better four-limbed fallen. Then again I could be completely crazy. Good theory.