If you have either of these situations: 1) a series of dependent database outputs (as in database-supplied auto-incrementing IDs needed for subsequent Inserts/Updates) or 2) writing a file that is used later in a workflow (as in a Run Command call or an R script), you need a way to guarantee tools that don't have outputs themselves (like the Output Tool) are complete. I believe this use case is fairly common.
If the Input tool had an (optional) input stream and the Output tool had an (optional) output stream, you could fully incorporate them into workflows, including use of Block Until Done. I know there are some alternative solutions to make this work, but they are not intuitive.
