In order to save ammo, I would heavily use melee weapons to kill low-level enemies (often by circling around, taking advantage of their low turning speed) if I knew that their attacks cause no considerable damage to my armor. The health is trivial to restore for free by sleeping in beds, but repairing gear at vendors is very expensive at least in the early game and your own repair skill limits your capabilities.
I don't know if blocking reflects all damage, but countering all attacks isn't necessarily easy in even 1v1 scenarios due to unclear animation. When facing three or more foes (let alone ranged attacks), it generally isn't viable.
Best Answer
There's a lot of incomplete information regarding armor degradation online and what I type here may be incorrect, but based on my research it seems likely.
The biggest issue is the Fallout 3 modding wikia (Bethesda's GECK) does not have information regarding Damage to Armor. They have a page on the formula fDamageToArmorPercentage but it has no content.
However, we know that Weapons break at a fixed rate per shot. For instance, Small Guns degrade at 3% of their weapon damage per shot, Big Guns degrade at 6%, Unarmed and Melee at 5% per attack, and Energy Weapons degrade at 4% per shot. Source
Given the way weapon durability works in Fallout 3 and knowing there is a Damage To Armor Percentage value it is possible that Armor degrades in the same manner: a flat rate of degradation per attack.
However, the default value for this variable is 0.350000, or 35%. Your character's armor does not lose 35% of it's health when you get hit so there is likely another formula we need to find. For reference, setting it to 0.00 means your armor will never degrade.
What we can infer, but not confirm, is that armor degrades at the same rate regardless of the strength of the attack. In other words, your character will lose an equal amount of armor durability regardless if they're hit by a radroach or shot by a missile launcher.
I am going to do more research on this and update this answer when I have more information.