Potential roadmap for multisite

Three plus years later, we’re still finding oddities in old MU code, so it makes sense to bide our time and only undergo renames when they let us shed dead weight. The side effect of keeping the barrier to entry high for multisite is not necessarily a bad one — even three years after merge, the product as a whole is still very weak (which is being kind).

Source: Potential roadmap for multisite: Subdirectories, subdomains, open registration, and domain mapping – Make WordPress Core