We also need this feature. At our events, customers sometimes register with their private mail addresses and order a newsletter in the process.
When we then merge the accounts, the newsletters go to the primary address (the business address). But for this address the newsletter was not subscribed at all.
So it would be better if the marketing mail would go to the address that also ordered the newsletter. Currently we have to keep both records for such customers.
@jaleonard19, you said in May 2020 that the feature would be coming soon - what is the current status?