While I do share some concerns here, I would remind all that the phrase "Perfect is the enemy of good" is considered a truism for a reason. First and foremost, the question should be "Is the game stable?" Not even "bug free" because that's pretty impossible in modern gaming, but "stable". If the game is stable and not going to crash to desktop every 30 minutes, that's already ahead of the game in many respects.
Right behind the first question is "Is the game fun?" Well, that's pretty subjective but I'd say as long as the empire size penalty issue on maps with a very large number of planets is dealt with, then yes it is. Broadly speaking.
The third question is, "Is the current feature set (as of Beta 6) a complete enough package for release?" Again, a subjective question, but, again, I'd say so. There are plenty of things I'd like to see, but sooner or later you have to release the damned thing. Since Stardock is saying it will be adding new features via patches, never mind DLC and expansions, I'm not too worried on that score.
Will some complain that they are paying for an "unfinished" game? This is the internet. People will always find something to bitch about. Especially on that wretched hive known as Steam. . (I kid because I love. )
Sure, having more time to fully test things for balance/fun purposes would be ideal. If it were me, I'd probably want a full six weeks hammering at the Beta 6 rule set to look for problems/exploits. But, then again, I don't have Stardock's bottom line to worry about either. They do have to release sooner or later. And there will always be something that could do with more looking at. Plus they have to worry about the schedule for their other games/their competition. Like it or not, but GC III does not exist in a vacuum. Even if it would be ideal if it were.
Still, as long as the game is stable, fun to play, and feature rich, that should be enough for release. Then they can spend the next seven years polishing and adding to it.