We actually had a chat about that (me and Undecaf and a couple of other users) on the Obsidian boards, so I guess I'll spare J.E. Sawyer and just copy-paste his answers :

me: I'm honestly not sure what is the design reason for this.

Josh : Because it makes the game boring against certain enemy types once you get heavy enough armor. With MinDam set to 0.0, eventually you can literally just walk around a horde of guys pounding you with low-end weapons and never take any damage. It doesn't really produce good long-term game play. Similarly, I don't think it produced good results in F1 and F2, where near the end game people would go round after round taking 0 damage until an armor-bypassing triple damage critical forced a reload.

0.2 MinDam feels pretty good to me. When a weapon's hitting MinDam, that's the equivalent of 80% DR, which is nothing to sneeze at. If some dude pops out with an SMG and starts blasting you in good armor, chances are pretty good that his (for example's sake only) 9 DAM is going to be reduced to 1.8 DAM. A weapon that maybe did 90 (for example's sake only) DPS now does 18. Sure, it doesn't make you invulnerable, but that's a huge reduction.

If you disagree and are playing on the PC, you are certainly free to tune MinDam, armor DT, or even armor DR if you are absolutely in love with the old F1/F2 armor systems.

Josh (talking about critical hits bypassing armor as an alternative method to MinDam) : I disagree. I think it is a really terrible mechanic because it relies almost entirely on random chance and virtually nothing that the player is doing. Fighting Navarro is probably the best example of this, as you and the Enclave trade nickel-and-dime body blows round after round until a crit wipes out (hopefully) an enemy or WHOOPS! You reload the game!

The discussion actually went on, but I don't really think there's much to add to these two posts.

(Hopefully copy-pasting from another board is not considered bad practice here. If it is, I apologize, but I prefer to avoid paraphrases.)

Insightful.

The whole point of random chance in RPG's is to provide an impartiality of events.

If your playstyle is "get killed and reload", it works out as a "what if I had done it differently" sort of scenario, and the PC's life takes a tangent where they step left instead of right... This is also part of the game IMO (part of many games in fact) ~Why make it possible for the PC to die at all one wonders, if this is the driving logic behind these gameplay decisions? PC death successfully avoided against serious odds, (including a few random chances to get killed), makes for a far more interesting gaming experience IMO.

(Where was this thread on Obsidian? )

Damage Threshold is in because any idiot can take Damage and subtrack DT to get the result.

But the thing is that there's really not that much you can do as a player to defend against the sudden critical hits that drain all your HP in one go, even if you wear heavy armor. I think that's the real point. That said, I don't necessarily disagree with you either because the random chance to get seriously hurt by a critical makes battles feel a lot more tense to play. Some people feel re-loading is simply an obstacle in games, and I can see that, but I personally kinda buy into the concept of the player character dying. It's something to "fear" in-game.

I see the logic behind the whole 20% of damage going through. I'm not sure I agree with it because a lot of it really will depend on how stuff like armor and enemies (and their weapons) are placed in the world. I do think the 20% damage rule should be lifted for some weapons, like a BB gun for example if it's in the game. I don't want to be able to shoot my BB gun (or be shot at for that matter) at a Power Armor and actually be able to do damage to it. But yeah, I think this is something that I really need to get a feel for how it actually is in-game because there are so many factors aside from the pure rules-side of it, like the possibilities of what enemies you'll meet where and when and all that.

EDIT: Also, the thread is the one for New Vegas on the Obsidian boards. There's only one I believe.

I'd like to think that's the point of criticals (not that they are instakills, but that there's very little the player can do to defend against them - and NPC's too). They, sort of, give everyone a chance.
And people seeing reloading as an obstacle... Well, it is. Whether or not it's a nuisance is everyones personal preference, but I find it odd if someone thinks of it that way in a game like this.

The thread is in the Computer and Console forum.