Yeah, people are willing to trade Smart over Rozier because he can score better, but thats about it.
Smart brings much more on the defensive end, which is great coming off the bench, and his ball handling and passing has become really good.
There are plenty of reasons to trade Smart over Rozier that have nothing to do with believing Rozier is better than Smart. Here are just a few: Smart is better and thus has more trade value; Smart makes more and thus can get to a minimum salary faster; Smart expires sooner and thus is do for a big raise sooner (and thus you might rather trade him than lose him for nothing if you aren't going to pay him); Rozier might be a better fit than Smart is with Fultz (I don't believe that, but it isn't unreasonable to think that way); Smart hasn't gotten better offensively and thus you might believe he is set while Rozier has shown signs of improvement and thus could still get a lot better.
I'm sure there are others, but I think you get the idea.
That said, I've been down on Rozier for awhile and for much of the year I felt he had almost negative value. At the end of the year, he played quite well at times though, which gave me a lot more hope going forward, but honestly I don't think he plays hardly at all next year without some major moves because I fully expect Fultz to take all of his minutes.