This needs to be in core as there a couple of ways to map using free & premium plug-in solutions and can be confusing.Make setting up a MS easier is progress and a thumbs up to Multisite development. I'm also concerned about how SEO & and even linking on MS is a bit of a hinderance to fast development. Often I have to develop on the subdomain/subdirectory url then switch to a url - then check like mad that no manual or links via plug-in content is still not pointing to subdomain url links. Ok this is done to some plugins not linking via the database (and then when the root url is mapped being intact.

if it was core it would be part of the environment and plugins & themes would be developed with this in mind. It would be great if plugins like Back up Buddy would finally come off the fence and support MS Back Up - i think it would if it was core. Hope it comes soon (ish)