The difference is in the taste.
Good UI tastes good while bad UI tastes bad.
But some people like the taste of bad and some people like the taste of good.
It makes no difference if you could serve what people like.
Update: 20 May 2022
Looking back at my answer from almost 4 years ago, I would now expound that bad UI design causes bad things to happen like users not being able to achieve their goals; users making grave mistakes; users getting addicted the wrong and unhealthy way. All these (and more) are examples of what bad design offers.
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