Rebellion has two groups of problems: bugs and game balance...
It is reasonable I think to say that all bugs (or at least the vast majority of them) should be fixed before the final release...SD clearly is focusing their attention on trying to stamp out bugs and make sure everything at least works, and I think that set of problems will be more or less solved by June 12th...
Game balance is different, however...first off, SD is completely, 100% incapable of making a well balanced game without banking heavily on player feedback...that is the point of a Beta, but a Beta can only do so much...
Really good balancing takes a very long time to accomplish (we are talking months and months and months of continual player feedback and vetting), and it just isn't feasible to postpone an official release on the sole basis that the game balance isn't perfect...
The concept of diminishing returns really comes into play here...SD can release a game June 12th that is 95% finished, or they can wait 3 months, get more player feedback, and release a game that is 98% finished...or they can wait a year, and then release a game that is 99% finished...since perfection is virtually unobtainable and close to perfection takes a really long time to achieve, it just isn't reasonable for SD to delay their release of Rebellion much longer than mid-June..
I'd like you to consider some other very successful strategy games such as Civilization V or the Total War Franchise...these are games with larger player bases, more MP, and far more resources than SD or Ironclad...it took months since final release for updates to truly balance these games (or at least get them where they are today)...in fact, Firaxis is still balancing Civilization V despite having an expansion pack slated for next month...
Even with all the resources that Firaxis or Creative Assembly have, they still have yet to solve all the bugs for all the OS and hardware configurations out there...it is truly a monumental task...
As it currently stands, Diplomacy v1.34 still has glaring balance and gameplay issues despite being out how long? And you think more time is magically going to balance Rebellion? SD will never have Rebellion perfect, never...and it comes to a point where waiting longer just doesn't have the return benefits to justify a postponed release....