We have no current plans to remove diminishing returns on crowd control abilities. The reason they exist is precisely so you cannot perpetually CC an NPC and buy your group as much time as required. While yes, many games have diminishing returns for PvP balancing, many also impose diminishing returns for PvE as well for the above mentioned reason (see WoW as one such example). EQ did not have diminishing returns to my knowledge; but they also had random breaks in CC whereas we do not.
WoW did NOT have dimishing returns on PvE CC. You could sheep a mob forever for example. The dimishing returns you talk about are against STUNS, which work differently than in Embers... you can attack and dmg a WoW mob while stunned and have it not break. The dimishing returns are in there, so you can't chain-stun a mob to make it completely helpless while being killed.
EverQuest did also not have dimishing returns on the most important CCs, the mezzes... how do I know? I happen to have played an Enchanter that had to continously mezz 2 (of 6 mezzable) RAID BOSSES for like TWO HOURS in Plane of EarthB (Rathe Council fight). Roots and charms could break early, yup, but mezzes were fixed-duration (54sec base without AAs), NO RANDOM BREAKS and 2 of them were even unresistable (Rapture and Ancient: Eternal Rapture). Additionally stuns were fixed duration too... what did we do? Get 2 enchs, 1 cleric and 3 mag/wiz into a group, pull half the zone and AoE them while both enchs kept everything permastunned... fun times.
Anyway... I have another question: Do the mobs completely stop checking for aggro while CCed or do they just no longer do their "do I still see my opponent"-thingy? Because if a mob that is stunned can essentially not notice what is happening around them, this might actually fix a lot of problems with healer aggro. (i.e.: 4 mobs inc, 3 stunned, 1 killed while using Triage a few times, healer = tank for remaining mobs)
And for clarification: I am not arguing against the CC-Aggro fix. CC should not be a mem-blur, I 100% agree on that. I just want to get rid of the damn dimishing returns that kinda make no sense in this game. Sta cost and especially cooldown of CC abilities, combined with random resists and reduced duration on Ashen and nameds are enough to balance CC already, without the extra step of DR. Best example for this is the Brigand 10sec stun... without DR it would be a very useful ability that would still drain the brigands sta and essentially remove his dps from the fight while he is busy CCing, but as it currently is... it is a crap ability not even worth the slot. Getting the mobs immune to the normal stun is worse than not using the new ability at all.