Many bugs are just not going to get caught by a small group of testers easily in a sandbox game, but with a huge group of players they will. I don't think the game would've been significantly less buggy if they had delayed the release by a month or two.
Very true. Sample size is a huge factor in catching bugs. Different systems, different play choices, different paths taken through terrain, different weapon combinations with enemies, different dialogue choices incorporated with different actions taken. The combinations are massive. Even with 1000 testers playing everyday for a month, they wouldn't nearly catch them all (would barely make a dent in fact). Take into consideration the price of those testers, assume 10 hour work shifts for 30 days at $8 an hour and we have ourselves 2.4 million dollars of testing. Compare that to releasing the game and gathering glitches from players who have payed them and you get profit and free testers compared to delayed release time and payed testers. Make money vs lose money. There is no contest on this subject matter from a producer stand point.