When using JGroups backends, the user might need multiple channels: one per index. If using an Infinispan Directory, he will need one more.
As seen on the below forum post, some people have this situation but even beyond that will deploy multiple Hibernate Search applications in the same container, and then it becomes complex to maintain isolation from all those UDP packages over the network. Not to mention efficiency is not as good as when you can share some protocols.
FORK being best explained by: