Actually it will be interesting if you sabotage them. Play a double agent style, doing little quests that grants reputation, then take their wealth.
Achieve liked reputation for the safehouse key, so you actually gain good armor for infiltration missions. Make Chief Harlon commit suicide, wich will weak the republic, because this way they will become even more corrupt; after this mission stealth kill the officers in Forlon Hope for Dead Sea and hear from Caesar how you turned Forlon Hope into a graveyard; don't kill all the Fiends leaders, so they are able to attack McCaran during the battle for Hoover Dam; make Boone bitter during his quest, so he commit suicide in the end (although he will kill an incompetent and lame officer, so i don't know if this is good

); "kill" Helios One, so no one can use it again; help Contreras stays as McCaran's quartermaster, wich is good for you, but bad for the NCR; and of course, blown the monorail.
After you decide wich faction you'll support, them you can attack their settelments.
My two cents, of course.