First, predicting how masses of humanity will react to something is impossible. You can say it was a good or bad risk for Bungie to take but until Halo 3 is released we'll have no idea if the improvements the beta release allowed outweigh the loss of hype and/or disappointment with a game that isn't up to graphics expectations.
I would generally think the beta was a good risk. It would be worse to release tidbits, build up a huge amount of hype, then the game disappoints graphically and/or has serious problems here or there. That could work with Halo since it was THE console FPS (the first one since Goldeneye four years earlier). It was also possible with Halo 2 since it was THE console FPS online game. Halo 3 doesn't have that luxury. If it slips in any way there are plenty of other great online console FPS'. It's in Bungie's best interest to do everything possible to make sure Halo 3 is as refined as it can be.
That being said, people are notoriously fickle. At first I though people would be rational about the beta multiplayer build since it's a beta multiplayer build. How silly I was, reading comment sections, boards, and forums at gaming sites the Xbots seem to be far angrier than I could imagine. Many of the Xbots talk about Halo 3 the way they talk about the Wii, it really surprised me. The concept that the gameplay may be worth graphics not up to GoW or UT3 (haha, Xbots upset PC games outclass their system) seems alien to them. Admittedly if the forum opinions of such hard core gamers counted for much the Wii would be dying a quick and painful death while a 360 sits in every living room. Then again such gamers make up more of the 360 fanbase than they do any other system.
On the whole I still think it was a good risk for Bungie, but we'll have to wait to see this September.