Single login is said to be coming soon, but what about unifying other things beyond just logging in?
From my experience as sysop at several Wikipedia sister projects (Polish: Wikibooks, Wikinews, Wiktionary; English Wikibooks), significant part of vandals are coming from Wikipedia, just after being blocked there. Another problem is open proxies: English Wikipedia blocks many of them, but this process is not synchronised with other language editions and sister projects and as a result spammers and vandals can play cat and mouse with admins there using IPs that are infinitely blocked on English Wikipedia since a long time. In fact, Wikimedia's open proxy policy is not working at all - blacklist on for example Meta is available, but there is a separate one on English Wikipedia and people just don't have time to apply all these blocks everywhere.
For me, it is logical that block of IP number should be common to all Wikimedia projects. Allowing this would have a large impact on number of vandalisms in sister projects.