The use case I'd propose is automatic end-to-end migration from one host to another.
I appreciate that this is something you might want to positively discourage.
Nevertheless, here's the reasoning.
In my opinion, having two hosting accounts with different companies at around 10 dollars per month can be a better bet than having one hosting account with a supposedly higher-reliability company at around 20 dollars per month.
But that's only the case if it's easy to migrate between the two hosting accounts.
It would be good (I suggest) to be able to click a button on Friday evening which automatically migrates everything from one host to the other, so that (allowing the weekend for DNS propagation) everything is up and running on the other host on Monday morning.
Then the next Friday evening, one can repeat the exercise in the reverse direction.
To this end, the Dreamhost API is currently lacking functions to Create_Domain, Create_SubDomain, and Create_Database.