FWIW the reason this was not the default originally is that you then get two projects for each origin PR which do almost the same thing, differing only in naming and whether or not they merge with the base branch. Some people may want this, but if computer time is valuable, you would be wasting a lot.
You can choose to suppress the branch project once the PR is filed, if you only want to build merges. That could be confusing too, of course.
Adding blueocean-plugin component as this is required for the behaviour we desire.