Thank you for the review.
I can see where you're coming from and I haven't migrated any sites to D8 except for testing. In D7, however, I think I have used MigrateDestinationTable (which is part of core migrate) at least once in every site. It seems there will be a situation where this will be needed. I know this is not possibly something that core would use but I think custom migrations most probably will.
It can be in contrib, of course, but the whole ecosystem is a little confusing right now with so many migrate_* modules. I didn't want to add another one for such simple and basic functionality. There was migrate_extras that did something like this and it was eventually discontinued.
I had been thinking of a migrate support library with common sources and destinations for D7. Maybe there could be something like that for D8 but I am not sure.