It seems to me that the Powder Gangers offer nothing at all. You just end up pissing off the NCR and Goodsprings (factions with vendors and other services) and at the end of the PG quest line the NCR raids the prison and kills them all off anyways. What's the point?

Am I missing something?

well, if you finish I Fight The Law on the Powder Gangers' side, they supply you with powder and stuff - handy for crafting ammo

Another advantage of staying friendly with them is that you wont fail I Fought The Law, because in this quest you can eventually turn your back on the PGs and fight with the NCR to get them their prison back and you can get some NCR rep

that's about it...

The game is set up so that you really can't get out of Goodsprings without starting to piss of the PG, then by the time you get past the first camp, they hate you and then you can't do all the options with the NCR/PG quest later on at Primm. Sure there are ways to get out of Goodsprings and avoid the whole mess, but the game almost forces you to deal with the PG right away.

However, other than that one questline there is little to the PG other than being targets.

Ironically, Vault 19 seems to never hear about this, but they are not aligned with the PG, but you would think there would be some solidarity with those folks.

I've only become friendly with them on one character (my Caeser playthrough), and I ended up becoming enemies because not all of them are friendly, but you can still lose rep with them from killing them, plus there isn't much to them besides a couple quests. Between that and the lack of Legion towns and companions, I think the whole "evil character" playthrough is limited.