The weight is really a silly question, as tsl points out. Bike commuters do not need to be weight weenies. That does not mean weight doesn't matter overall. We should add weight judiciously. The utility a dynamo adds is huge, and the weight is, by comparison, tiny. A typical dynamo system probably weighs a little more than a typical battery system, but the difference is probably less than 500 grams. Then consider that some of us carry spare batteries when using battery systems, and the weight difference is probably completely insignificant.
I'm not a religious nut about this: I use both kinds of light systems on my various too-many bikes, but I like the dynamo systems a heck of a lot more.