I'm terrible at DOOM but i've taken down a Cacodemon with a pistol pretty easily. The pistol fires a large amount of shots per second, really; just not huge.I could probably take down a single Mancubus with the pistol if I had a soul sphere when I was done with the battle to recharge my health (and I had armor.)
The pistol is also good against Demons in a large room, not better than anything else, but if you start a level in a large room up against a few Demons just back away and keep firing!
Finally, the pistol is awesome for sniping. Take out enemy targets like Imps, Sergeants, Demons, Hell knights (I'm not kidding, it takes a while but It could work) and Cacodemons from a window. This is particularly good on Demons - even if -fast is enabled, making the Chainsaw useless against Demons, the good ol' pistol can take 'em out via sniping since their only attack is short-range.
From my POV the most useless weapon is a Chainsaw- but only when you have a chaingun and full ammo. The Chaingun does almost as much damage per minute as the Chainsaw, but can do it at range. Also, the berserk fist is nigh useless if you have a Chainsaw, or a supershotgun and a lot of ammo. Obviously the non-berserk fist is useless - but come on that doesn't even count!