For the Legion, would it make sense to make the Powder gangers strong so that they can disrupt the NCR even more than they do already? I believe that even a full strength powder gang would be nothing more than a slight inconvenience once the Dam fell, however the fact that a full strength Powder gang could disrupt the NCR supply routes and instil fear in to NCR troopers means that they are demoralised behind their own lines. Or do you believe that a Legion recruit should destroy the powder gangers as soon as possible?

From a Frumentarii point of view helping the PGs just so they can knock the NCR down is smart, but you'll need to put them both down eventually, since Legion will never ally with the likes of the PGs.
As a Legion recruit you would be picking off their outposts, and for brownie points wipe out the NCRCF. I assume you're playing more foot soldier so this is likely what you'd do. For stealth and more tactical approach you'd do the first suggested idea I said.