The three families can be useful as spies. Securitrons aren't going to be very good at infiltrating an NCR or Legion base and stealing intelligence from the enemy.
Neither are greasy-haired hipsters with a habit of saying "Ring-a-ding-ding!"

Remember Benny's attempt to infiltrate the Legion?
Still, my money's on the Securitrons. Vegas' military can't be demoralized, they can't be weakened by cutting off trade lines (they don't need food), they can be absolutely relentless with no worry of fatigue or needing sleep and they can be rebuilt and repaired. You'd need a LOT of impulse weaponry or expert snipers to even leave a dent in the army. With self-repairing Securitrons, all Mr. House has to do is send them in waves and have program them so the damaged ones fall back so that they have time to repair before sending them to the front lines again. Wave of securitrons, 6 missles fired by each, reload or fall back if damaged, rinse and repeat. Healing a human on the other hand, doesn't run like clockwork. The human can't be told exactly when he needs to back off, he needs an escort off the battlefield and his wounds may never heal.
And what's all this about "the NCR can build up and research a way to beat the Securitrons." And Mr. House can't? Is it just me or did Mr. House not seem like the biggest freaking genius in the entire game, and you think a society that's just now recovering old world technology (farming, power plants, vehicles) is going to out-tech a genius who founded RobCo,
warded off atomic bombs and survived the Great War?