Seeing that almost every late-game enemy has considerable DT, aren't they essentially useless?

They're great for killing everything else. Not that there are all that many high DT enemies, plus you get the Shotgun Surgeon perk.

Riot Shotgun works fine for me, even at the battle of Hoover Dam. Caravan Shotguns do become a little useless after a while.

You also have slug rounds.

Another one is to deal with Cazadors.

Slugs and Shotgun Surgeon, booyah.

I actually defeated Lanius using an ordinary shotgun, because like an idiot I left my gauss rifle that would have blown his head off at home.

Maybe it should have a -5 item hp against enemy armor for each pellet?

Shotguns' use is precisely as a close-range weapon against high DT enemies.

Handguns and SMGs do not have DT-reducing rounds at all, so they're not a very good choice against high DT opponents. .44 Magnum revolver, Hunting Revolver and Ranger Sequoia can be used with DT-reducing handloads, but they reduce less DT than Shotgun Surgeon and they're less powerful than shotguns. Assault Carbine and LMG do, but their per-hit damage is low, their critical hit multiplier is low, their condition degrades extremely fast and they are expensive to repair, so their routine gameplay use is difficult.

Shotguns are meant to be used with the Shotgun Surgeon perk which reduces DT by 10. Their per-hit damage is up there with the Brush Gun, and it is delivered in single shots, which means that the shotguns' degradation rate to damage output ratio is much better than that of automatic weapons. With the exception of the Riot Shotguns, all shotguns can be fixed with BB guns, Varmint rifles and other cheap and plentiful stuff with the Jury Rigging perk.

Actually, rapid firing beam bags may kill them faster

Yup.

If you have Dead Money, "And Stay Back" makes Shotguns that much better.

Dinner Bell + shotgun surgeon + slugs + and stay back = everything you want dead, becomes dead. I've always wondered how trigger discipline would effect shotguns, but i've never took it so I dont know.

Now that you mention it, i'm wondering too, but i never made a character without it so i don't know either.

Wish i had NV on pc so I could test it, that extra 20% accuracy would make shotguns deadly.

-Not all enemies have huge DT numbers. Few have more than 15 now.

-Shotgun Surgeon + And Stay Back! = Cheating with buckshot. ~10% knockdown chance x7 pellets.

-Free-Aim sneak attack criticals can still roll large damage numbers. The extra damage is x7.

-Still the best CQB panic button in the game. Riot Shotgun can cause lots of pain quickly outclassing weapons that weigh more.

My cowboy characters carry around a Leaver-Action shotgun for dealing with weak enemies. The weapon and 85 rounds of magnum buckshot comes in at 8.525 Wg and can hit quite hard if needed. I cleared out Vault 34 (has buffed ghouls) using one with little trouble.

I don't really use them much. I figure if an enemy gets close enough for me to kill it with a shotgun, I can kill it better with Knock-knock, a Ballistic fist, or the Cram Opener.

With slugs and Shotgun Surgeon, Dinner Bell becomes a potent rifle-shotgun hybrid.

More about bonus sneak base damage multiplier. Critical damage of shotguns is about the same as the damage of each pallet.

shotguns are great when your in trouble and need a fast way out or just wanna make a mess
but i like shootin from a a distance with rifles takin em out before they know your there

Also, shotgun ammo is far more common than .45-70 rounds.

Not really, Quartermaster Bardon always has 500-700 rounds of each.

Providing you can find him. Plus you don't have to go through multiple load screens to get shotgun ammo. Also, if the NCR doesn't like you, good luck getting to him much less getting him to sell to you.