You asked whether use of Underscores would facilitate Theme approval for wordpress.com, but I can tell you from experience reviewing/approving Themes for the official wordpress.org Theme Directory: when I come across an Underscores-derived Theme, I am able to perform a much less-rigorous final approval audit. I don't have to worry about pretty much any of the template and template-part files, because I know the markup started from a compliant state. So, yes: I would say that use of an inherently standards-compliant starter Theme such as Underscores as the foundation for a Theme does facilitate review/approval of that Theme in the official Theme Directory.

Now, if we could get a single, consensus options framework, I would be a very happy camper.