I think there is a difference between using your negative emotions to foster change and wallowing in them. Left alone, and experiences without judgment, negative feelings resolve and go away on their own, and as you said, they can make you feel passionate to change things for the better. However, holding on to those feelings is not conducive for any change as you are simply attaching yourself to a situation.
I do agree with you that avoiding negative feelings is very very bad, but so is keeping yourself attached to them. For me happiness is not some emotional state I need to hold on to in fear of losing it, but the "echo" in the background that reminds me of how amazing life is, even when it gets hard and may "feel" bad.