So trading away some AC (which you can get back, except it's irrelevant anyway so you needn't bother) for some damage reduction makes you take more damage, not less.
And I suppose if you do recover the sacrificed AC, so you have the AC and the damage reduction, you're taking more damage twice, once for AC that doesn't matter and once for damage reduction that doesn't matter.
Interesting.
Except that you don't actually do that at all, because you either have an AC of x and a DR of x - 5, or you have an AC of x - 5 and a DR of x. You can replace 5 with any other number you like, it does not change the fact anything with a 1:1 PA does the same damage, anything that PAs harder does more damage, and anything that is not a physical attack does the same damage. Therefore, you take the same or more damage in absolutely every single situation.
Next you will inform me that math is hard.
As for the value of AC, no it is not because of weenog's fail. It is because you can pop a 50% miss chance on the cheap, which has the same end effect as an AC good enough to only be hit on an 11 or better. But is much cheaper. So if you can't get yourself to the point where enemies hit even less often, you're better off ignoring it entirely, getting some miss chances, having the same or better defenses, and lots of spare cash.
Also, anything less than Heavy Fort is worthless.
Why do you think that?
I like medium fort at a +3 bonus; a +1 (who would ever get more) heavy fort armour is around 16k gold, but still stops 75% of all criticals.
I find that quite useful. Yes Heavy fort is better, but it also costs 20k more, which it will take two levels more to get.
Moderate Fort, you mean?
And aside from the fact you are getting it to stop luck from shafting you, not to make it do so less often and therefore sub 100% defeats the point? If the DM does not allow upgrading, Moderate Fort is wasted cash. If he does, the level at which you get Moderate Fort isn't that much closer than Heavy Fort, so there's still little point. The only D&D related context Moderate Fort is useful in is DDO, for Warforged characters. Mostly because they houseruled that 75% to stack with innate 25%. Therefore, you get the mandatory immunity 4 levels sooner, and also have more options as to how you get it. A common caster item has Moderate Fort as an incidental benefit, for example. So that's slot savings. But RAW, when discussing tabletop D&D? Forget it.