The damage over time of Plant's AOE Immobilize is significant. It is significant enough to slot for damage and use it in that capacity on a regular basis (after the foes clump up a bit). Therefore I would advise you to cast the Immob anyway.
As for whether you can get drops from foes that are defeated when they never take damage from you, well I don't know. I always do at least some damage to everything.
Lewis
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I'm pretty sure you do still get drops. My Plant/Fire fire dom has died lots of times after casting seeds and iirc insps have popped into the tray while laying there enjoying the fight club.
One thing I've been wondering about while farming with my Plant/Storm...
If I lead off with seeds of confusion, and the killing blow for a particular mob comes from another mob, do I still have a chance at a drop? Just wondering if I might get a higher drop rate if I immobilize everything.
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