The two types of D&D designers I curse the most are those who stand in the way of the silver standard and those who stand in the way of replacing AC with DR. After all, it isn't like armour makes you less likely to get hit... it just makes it less likely that you get damaged.
Armour as DR is one of those ideas that keeps coming up, is really nice in theory, but doesn't work very well in practice.
The problem is that even if it worked as intended, it still adds a further unnecessary step to combat. Just now, the process is:
- Roll to hit, and compare with the fixed AC.
- If a hit, count up the damage.
- Subtract damage from hit points.
When you switch to a DR system, you get at least one more step:
- Roll to hit, and compare with the fixed AC (of Ref defence, I guess).
- If a hit, count up the damage.
- Subtract Armour-as-DR from the damage.
- Subtract remaining damage from hit points.
On a single attack, that extra step doesn't take very long, but when you're dealing with dozens of such calculations per session it very quickly adds up.
And that assumes the simplest possible implementation, where DR just subtracts from damage. However, the secondary effect of Armour-as-DR is that it opens up a design space, that the designers can't help themselves but to fill. And so, we get the "Iron Heroes" solution where armour gives
variable DR, adding an extra dice roll every time. Or the WFRP situation where some weapons have 'penetration' allowing them to bypass some or all DR (adding yet another subtraction to the process). Or similar.
And, unfortunately, making it modular doesn't really work - that was tried in 3e, but when Armour-as-DR was used it gave a sudden and huge boost to the already-powerful Power Attack. If it's going to be used, this is really something that needs to be built in from the outset.