I'm not arguing that he's been good; he irrefutably hasn't been overall and in an ideal world, I wouldn't make this trade. In the predicament they're in, considering what they're trying to do and the positives about him I've mentioned, I would.
Also, he's just 22, is one of the rare players who can credibly guard the best big wings (Leonard couldn't do anything against him earlier this season and it wasn't "missing shots he normally makes"; it was not getting shots he normally takes) and context is everything for 99% of players. Suffice it to say, his has been far from ideal.