Probably 2 things:
1. anytime you can turn two 50 somthing picks into one 30 something, it's probably a good move. Let's be real, Semih kind of actually sucked. He made a good impression because he sucked less than expected, but he was a major net negative this year. For the future? We'll see.
2. If you can open up roster spaces by getting a free second round draft pick, why not; and Luke/Semih were definitely the guys who would be cut for a buyout candidate.