I recall trying to explain to Celestalon some of the issues with Demo in WoD's development, I think it was AoE specifically, but he wouldn't have any of it until some others backed me up on it. We saw what happened to the spec after that. The developers often don't know the intricacies they create and base a lot on things 'working as intended'; so miss a lot of unintended consequences of their design. It's a huge reason why they're looking to simplify things.
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And this has a knock on effect at lower levels and in pugs, where people would rather bring someone else than a Warlock because the notion that they're bad permeates the entire game, and we all know that 'sub-optimal' becomes 'bad' becomes 'useless' very quickly in the perceptions of the wider playerbase.