As anyone who works in software developemnt will tell you (to paraphrase) - no software survives contact with the enemy... eeerrr... end user.
Are there going to be bugs in the released version of Leopard? Yes. The only question is whether they will be "show-stoppers" on a per-user basis. 10.5.0 will be followed by 10.5.1 (which will fix some "last minute" things that did not make it into the gold master). 10.5.2 will (hopefully) fix most of the issues that come to light when Leopard "goes live".
A cautious user will of course continue to use Tiger 10.4.11 (or 10.4.12?) until Leopard's replacment is released - and then upgrade to Leopard 10.5.14 (or whatever).
Hence the reason you do a wider "release candidate" of a major software release. I mean really at this point the whole ooooo we have top secret features is beyond played out. Apple should drop a preview version of leopard for $10 which should cover the pressing, and shipping of the media easily. Build a small app into the RC for easy feedback, and have the OS expire in 90 days. I'm sorry but there are bugs in Tiger that are still present. USB bugs should have been squashed on day one of release. The behavior by apple borders on the unprofessional IMHO. IMHO you WANT to test the living crap out of your OS before the launch.
For as buggy as Vista is if you look at before they released it to the masses as a RC and after you will see that there were substantial gains in quality and stability. Again there is simply NO reason for Apple not to do this once the code gets more polished.
Apple has a huge fanbase who is more then willing to beta test. Go look at the bittorrent sites and see how many people have already DL these laster builds of Leopard. There is no reason to fight this. It a tool that Apple could use if they were the most arrogant company on the planet, which I believe they are. I'm certain it comes down to Jobs and his planet sized
EGO. My guess. "Well we don't want to show the masses something that is buggy so we are keeping it behind closed doors." Its a load of BS but its BS that I'm certain Jobs is delusional enough to believe.
It is true, because such a limited number of beta testers for what is a huge piece of software will mean that bugs wont be exposed as quickly.
I personally don't mind too much. I will still purchase Leopard on day one. Part of the appeal of following Apple is the secrecy. You lose a lot of that if Apple start announcing their products 3 years in advance and having huge beta testing periods. The bugs will be worked out for the most part by 10.X.1/10.X.2. People who run mission critical processes should not be installing a new OS that is bleeding edge anyway.
If you want secrecy go buy a box of cracker jacks and pull out the prise. I want a solid OS to work on. This isn't a game. And frankly Apple CAN keep its secrecy. But 6 months before release would it really kill them to open the flood gates to us nerds?