I expect that some site builders will want to directly place a block instance in a Twig file (like page.twig.html) without making a theme-level region and/or using the Block UI. Combined with Twig's "extend" concept, direct block placement could be a very powerful way to do layout variation.
Here's a proof-of-concept gist that only deals with blocks. It could be abstracted to deal with entities of all types: https://gist.github.com/stevector/42c8f98a424fc0a0e1ee
Should such functionality go in core? It could be a very small contrib project.