First, let me express how much this tread rocks.
It rocks as much as BillZBubba is rocking claws.
No really, it is a staple of numbers study.
Now, let me correct a tid something I'm reading a bit too much. Resisting enemies don't make resistance debuffs less relevant. Nore more. They are just the same.
Even the devs stated that (can't remember where, nor who) "Resistance resisting resistible resistance debuffs" (or 4RD) was actually meant for resistance debuffs to stay relevant whatever the resistance of the debuffed was. It will have the same "feeling".
Take 100 damage applied to a 25% res mob. 100 is a nice round number but it works with any. You inflict 75 through res. Now, add 20% res debuff on it. It's only 15% now, because of the resistance (4RD). So 100 damage is not 75 but 90. 90 is 75+15, and 15 is 20% of 75. So you increased your damage by 20%.
Lucky result? Let's look at 50% res. 50 net damage without debuff, -10% net res debuff, so 60 damage with debuff. 10 more damage, that is 20% more damage than before the debuff.
I could do the mathematical demonstration in nice Word or LaTeX, but you math-inclined should get the reason why it works without it.
Bottom line is, don't scoff at any number of -res being meaningful "only" against the non resisting pylon. It's still doing its part. More enemy resistance is just more -res cap (because it's harder to reach -400% through the resistance). Also, it means the res debuff will be more meaningful, because the mob is gonna survive longer instead of dying due to the damage of the attack that debuffed!
Now that means that, when soloing content, whatever gives you the least kill time is always the king, but in group, where one alt is the minor part of the team, resistance debuff is capitalized by the whole team. Inflicting 20% debuff means "you're" inflicting 20% more the dps of the whole team, that's, for 8-man teams, 1.6 additional player base DPS. In 4, it's still nearly a full player.
Though I guess this was not doubted anyway, just being as clear as possible for the neophytes that may pass. Sorry if I sounded pedantic. Thanks for your attention.