From the (un)-archived discussion at meta (http://meta.wikimedia.org/w/index.php?title=Wikimedia_Forum&diff=next&oldid=3619272), [[User:JohnnyMrNinja]] writes (in part):
I am proposing that account unification be completed for all eligible accounts without requiring the user to take any additional steps. This would make UL the rule rather than the exception that it currently is, and bring us closer to the goals of universal watchlists, recent changes, interwiki page moves, etc. This would be especially helpful on Commons, which has so many images that were originally uploaded at another WMF wiki, enabling better attribution without interwiki links. I propose that it be carried out as a one-time process rather than a continuous automatic software process, allowing users to still adjust ULs as they see fit. [...]what I mean by "eligible accounts" was all accounts without existing conflicts, as these can be taken care of by an automated process. Accounts that have conflicts would be unaffected by this specific proposal. Conflicts could not be solved by any automated process, as each case would be different.
Looks like it has enormous support on meta. I'm not sure about the ins and outs here, so I'm going to add people who I think know more as well as post this bug to wikitech-l.
T41817: Migrate to SUL all non-clashing accounts
T38939: User name blacklist should be managed globally to avoid SUL fragmentation