A half link chain will stretch more than a standard chain due to the fact that each side link has a `z' bend in it, these try to pull straight under load.
Not just bike chains, I used to work in a materials handling company, we tried big, chunky half link chains on conveyors, as it would allow closer pitching in certain applications, but we sacked the idea due to stretching issues, and these were about 4 times bigger than bike chains!!
MM