Most enemies in hell actually deal less than 100 average physical damage (early Hell monsters might deal 60 per attack, while those deadly archer packs are only hitting for 80 per attack). Really, on the truly high-end monsters (Venom Lords, Minions of Destruction, Hell Cows) can hit for 140 average physical damage. As a result, in 90% of situations, one point of PDR will reduce more damage than 1% DR.
The thing is, though, PDR becomes less effective in the really, really dangerous situations (might enchanted Frenzytaurs, anyone?). As a result, while PDR is more effective 90% of the time, the 10% of the time that %DR really shines is the most dangerous 10% of the game. For that reason, I have come up with a completely arbitrary conversion- generally, I consider 1% DR to be worth as much as 1.5 points of PDR (so 50% DR is roughly equal to 75 PDR in terms of survivability). Obviously the big advantage of PDR is that you can get more than 75 PDR (pretty easily, at that), while you can't get more than 50% DR- that's why I tend to prefer PDR rigs to %DR.
Having lots of PDR doesn't really lessen the effectiveness of %DR, although it seems like it would. If you have 0 PDR and a monster normally kills you in 30 seconds, then 50% DR will boost that to 60 seconds of survival. If you have 50 PDR and a monster normally kills you in 30 seconds, then 50% DR will boost that to 60 seconds of survival. In both situations, 50% DR doubles the length of time that you will survive, so there's really nothing wrong with mixing the two.
If you have any EDR, though, then PDR gets REALLY effective in a hurry. If you are a Necro or Barb, PDR is worth so much more than %DR that it's not even worth considering the latter. With decrepify, 50 points of PDR are as effective as 100 points of PDR without it. With a super high-end Taunt (slvl 45 or so), 50 points of PDR are worth as much as 500 points of PDR without the Taunt. As I said, it gets really powerful really quickly. This mechanic is the reason why my absolute favorite barb builds all rely heavily on Battle Cry and PDR.