If a group of armed players can hold against the entire guard population of a town then they should taste their sweet 15 minutes of victory before guards respawn.
Sure, it would be extremely easy to attack Junktown at night or maybe VC if they avoid the turrets but the NPCs in guarded towns are strong enough to give trouble to an armed gang.
Plus... What's the worst a group of players can do after killing a bunch of overpowered guards that'll respawn in 15 minutes anyway?
And don't use that "Oh PKs will just camp guarded cities" argument, there's at least 50 guards in NCR and Hub, the forces in Junktown and VC are quite strong too.