For me, the beta was a chance to learn how to not suck while moving around Metro. And learn to use camptastic tactics like PKP+bipod in the smoke on B approach 1st MCOM. Back when it hid the muzzle flash so no one could even figure out where you were. Ahh, those were the good old days.
Even more important was being able to hook up my wired 360 controller to my PC and playing the beta on it. Once I set up the controls to my favorite southpaw style, I was able to learn how to fly. Didn't take too long, but the lessons learned were invaluable so on Day 1, I was able to hop in the sky and unlock stealth, flares, rocket pods in short order.
So I would say the beta is extremely useful in learning the mechanics of the game. Graphics shmaphics. And I don't care about 64 players vs 24. I could be thrown in a game of 4 on 4 and the mechanics are the same. So approach the beta like you want to learn HOW things are different.
This is why you'll probably still see me playing the BF4 beta when it's out and will be like "OMG ZATHS COMING". Nope, just seeing how the mechanics compare to BF3