I've studied the case. From what I can understand:

Merge and refesh cascade loaders are special loaders involved when:

• We merge a detached Entity
• We refresh an attached Entity (dual case)

The main benefit consist in the application of CascadeEntityJoinWalker to create the join and collections on Loader instance.
We can reduce the amount of query to perform in order to perform merge or refresh operation.

But for Hibernate OGM, where we do not traverse the associations using query, I think that now it is not useful.

Anyway I prepared a PR that will open the path to future integration, fixing some methods and adding a test focused on merge and refresh, on Entity itself or from a parent.