This has been bugging me for a bit, but it always felt to me like Fallout DLC always missed the golden opportunity to add companions in game using NPCs added in the DLC. Let me give a few examples from 3 and NV.

In 3 we had The Pitt, Mothership Zeta, Anchorage, Point Lookout, and Broken Steel.

Mothership Zeta: This is obvious with your fellow prisoners being viable companions, it wouldn't have been hard to have Paulson, Soma, Elliott and even Toshiro to be companions, though Toshiro would have needed some working with.

The Pitt: This would have been a good one with you finding Wild Bill in the Mill albeit alive and he helps you through the add-on through request from Milly. And if you turn on the slaves then you get Krenshaw as your companion per Ashur's request.

Anchorage: This one is a bit harder but say there was a room where the Americans froze some of their highest ranking or decorated officers and it is opened along with the weapon storage at the end. You could find a ghoulified Benjamin Montgomery who you can convince to help you through explaining what happened.

Point Lookout: This would probably have to be the hardest one as you have the option to kill Calvert or Desmond and Calvert dies either way. My best guess would be have Nadine be a companion because she wants a little adventure on the side past her newly obtained boat.

Broken Steel: Not really much you can do as it's more of an extension of the story in the base game.

In NV we had Dead Money, Honest Hearts, Old World Blues, and Lonesome Road. This would have required more work due to perk changes and possible quests.

Dead Money: This parallels Zeta almost with three other captives helping you. Simply enough they would have had different endings and could be found in the Mojave, such as where Dean goes anyway if left alive. It would likely have small side quests like getting Dean a job at the Tops, Christine reuniting with Veronica, etc. The hardest part would have been Dog/God as they would have changed quests depending on the state the player left them in.

Honest Hearts: You would add Follows-Chalk, Waking Cloud, and possibly Joshua Graham. They would be simple additions, Waking Cloud wants to make connections with the Mojave for her tribe and Follows-Chalk decided that if he wants to travel he might as well be guided by you. Graham would be the hardest to do because of his being against Caesar, though he could just be an NPC and not disappear like in the current DLC when it finishes.

Old World Blues: This is the hardest one as there are no real companions and Roxie is left in one place never leaves it. So there probably wouldn't have been one for this as the new living space you get would suffice.

Lonesome Road: This would be strange as you already have an ED-E in the base game, however it would also be possible for this ED-E to be upgraded after the story and he stays with you until fully ready to continue to Navarro. And the last one is Ulysses who would need to survive the final encounter and be convinced to explore the Mojave with you as you follow your own path. He would need lowered stats as a follower to not be extremely powerful, however.

Anyone agree/disagree with this?

Nope, there was a reason they didn't add them in. I'm not a tech genius, so I can't properly answer it, but it's something to do with game files or stability or some such.

No it's not about technical reasons. It works fine if you mod it in. Mostly it was about balance and budget I think. DLCs are hard to balance because the player is already at a higher level. Additional companions would make it harder to find a good balance. The same is for budget DLCs don't get that much development time as a main game and last but not least getting an actor again for a DLC role isn't that cheap,

Ah, I'd heard it was something about it being stability of the companions in DLC areas or being too painful to map DLC zones for companions and was just parroting what I'd heard.

I would definitely like to have had Dean Domino, Christine, Dog/God, and Joshua Graham as possible companions after the DLCs. Even without additional quests.

Never gave much thought to extra companions in F3, although Desmond would have been interesting. Toshiro could have been an ED-E type of companion,

speaking Japanese instead of "machine".

The main reason for this in New Vegas is the story. If you did not have a DLC companion with you, then the story would not have been the same. Also, they did limit you to one follower in the game, due to scripting overhead, so that means you could only have the DLC companions. In FO3, OA is a simulation, so why would you have a follower in that? Although, you do have them to an extent in the final battles. MZ has NPCs that you work with, so that has followers, the Pitt does not lend itself to followers, because the Arena is something you need to do on your own. PL, well, that was just supposed to be a challenge and the swamps are scarier by your self.

Sure, all these design choices could have been different, but I don't think the DLC experience, as they were made, would have been the same with stock followers.