I'm wondering about something (ain't I always?)

FNV's main game had to be designed to be playable without the DLCs. That's fine and reasonable, even unavoidable. However, the DLCs raising the level cap, adding new perks, weapons, weapon mods, ammo and crafting recipes make the game easier. I'm not easily seduced by the "coolness" factor of having bigger, better everything, so I'm currently waiting until the opinions on the Gun Runners' Arsenal reach a point of consensus on how it impacts the game.

It's not so much any specific single improvement as the cumulative effect of them all. When I downloaded Dead Money, HH and OWB, I didn't notice a meaningful increase in the enemies' toughness- with the notable exception of Legionary Assassins- but my character's capabilities have certainly grown. The game feels more and more like a cakewalk.

Should each DLC come with a built-in boost to the enemy toughness?

That's because most of the enemies you find in the base game are not lvl scaled, so they don't lvl with the player, unlike DLC enemies.

I like it the way they did it.

FO3 did it the way you suggested and all it did was make travel an absolute pain. Every two steps, an absolute bullet-sponge charging at you. It got old. FAST.
The way FO:NV did it, if you want a tough fight, go to Lonesome Road, Big MT or Zion. Those that want a challenge know where to find one and those who don't won't have to fight a Deathclaw with 20,000 HP every two feet.

Yes DLCs break the game. The only way to balance it was to stop the levels at 30, or max 35. Look at LR, enough ammunition for next 10 fallout sequels.