Think of it this way, magic blockades can cause grouping of mobs which will then make it esier to break, this way elment is removed and they keep going on their way... INTO THE GAS TRAPS.. TO BE KILLED BY HARPOONS!
I'm surprised you're all missing the biggest reason why a darkness trap is more effective than a magical barricade. Granted, I am still debating internal on the DU costs for it. However, a darkness trap can strip the immunities right when the enemies get in range of your fireballs, or other defenses.
This means that normally fire immune Ogre that gets to pound on your barricades, can die before they get to the barricades. In addition, those evil Poison Immune Dark Elf Warriors that rush through your ensnare aura will have their immunity stripped before getting to the ensnare, meaning they can't ignore your barricades now like they do before.
I personally am looking forward to this change, and will be curious to see just how well it will interact with alternative setups. Another thing to consider, now that traps can strip immunities imagine a Darkness > Inferno > Gas > Prox > Spike trap setup. This means first they get stripped of their immunity, now they're taking full damage from inferno, in addition none of the enemies can ignore the gas trap choking effect (except ogres), and anything that gets by the inferno damage will end up taking full damage for not just the Prox but the Spike trap as well.
Why are people even complaining about the 5 DU being steep? That's one less electric Aura in exchange for more offense/defense (depending on how you look at it.) I'm sure people can incorporate one less electric aura into their builds...
Yes they're useful, but they still in the end hurt you for using them in the long run.
Darkness trap + spikeys should make mobs attempt to run past spikey (for melee enemies) but keep on getting hit? AND loss of immunity. Sounds good.
I haven't used darkness in a long time, but I do believe that this change is actually a benefit to the darkness because currently, it just is not worth it. You can blow them up faster then waiting for them to move on to another target.
Stripping immunities is particularly valuable. But for 5 defense units it might be too expensive unless it effects more then one target at a time. If it does effect everything in range, then 5 points will be a pretty good value.
Otherwise it would need some pretty big additional changes. Much higher trap count, or better reset, or some other effect, or some damage.