That's not what any of my beta experices have been like. All of the betas I've been in since the Halo 3 F&F beta have just been about playing. It's not like the employees from Bungie that I was playing with were sitting on their mics telling me to "stop playing and start testing for bugs". They just had me play. Same goes for Halo: Reach, and the dashboard previews.
The point of betas is public testing, yes, but it's not like they just want you to run around testing bugs.
Exactly. I was a part of the Halo 3 BETA as well, and the devs that were on mic with us just wanted us to play, and comment/keep record of any bugs/glitches/gameplay problems that we came across while we played. This way, the more detailed description of the problems that they have, they can pinpoint and fix better. It also involves suggestions of bettering the gameplay by suggesting improvements, damage/effort ratios, etc.
It's a lot of fun, but also can be very informative for the devs as they get the feedback from their very own players.