No it affects healing output as well. I took a huge bit to my healing once just to see how bad it would do it. My cure I went from 960~ to 340~
You don't want that debuff on a healer.
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Why is there still no statement from a Mod/GM/Rep on it?
What I have read from people doing the math is that 1 MND equals about 5-6 DET.
The Titan debuff is a weird one. It says "Determination Down" on the tooltip info but looking at my stats while it's on my determination is exactly the same. It also has no effect on healing potency.
Personally I think the description is wrong and it's just a massive attack/magic attack down buff, there's no way that lowering determination would reduce your damage by 80-90%.