I’d like to call out a superb blog post from Tim McMichael on this topic. To quote Tim, this is a topic, which is commonly misunderstood, and the benefits include the understanding of:
Why do you disable the MAPI network from log shipping activities? Remember that if no other network exists in a DAG to replicate log files we will utilize the MAPI network for log shipping. If the MAPI network is replication enabled, then when the replication service is choosing a network to perform log shipping it considers it at the same weight as identified replication networks. By disabling the MAPI network it is no longer considered at the same weight and therefore all initial log shipping activities are balanced between the enumerated replication networks.
Have a look at the full post here.