Gazimoff from Mana Obscura writes about how beta testing has changed, moving away from bug reports and using heat maps and data mining instead. If a large percentage of beta testers don't finish a specific quest, that tells you more than sifting through bug reports complaining about quests, many of which might be perfectly working as intended. Furthermore I can totally see the interest of Bioware to want to test if their servers can withstand millions of people, seeing how this is their first MMORPG and problems in that area are to be expected.
What is a bit weird is keeping up the NDA while having so many testers. Chris from Levelcapped points out that if you tell a secret to millions, it isn't a secret any more. Thus keeping information away from possible competitors can't be the reason for the SWTOR NDA. The general assumption is that Bioware marketing is using the NDA to better control either the hype or the negative opinions on the game. I'd say that strategy is backfiring. Some people simply don't care about the NDA, because they think that nothing bad will happen to them if they break it. Others use the fact that the NDA is still up to cast suspicion upon the game.
One thing which is certain is that the meaning of the word "beta" has changed. Google's GMail was in beta for 5 years. Minecraft is being "released" this week. Allods Online was in beta until version 2.0, with people already paying for items from the item shop long before "release". And many other MMORPGs obviously used betas as both marketing tool and server stress test tool. I've been in betas which didn't even *have* a bug reporting tool, bug reporting in betas is so last century.