Maybe I'm wrong, but my understanding is that proper zippering is simply waiting until the lane ends to merge. You align the right side of your car with the curb and let it weave you into the lane at its own pace. It doesn't necessarily involve speeding ahead to get to that point, although certainly lots of people do that. It minimizes the jostling and speeding up and slowing down that people do to try to actively find a place to fit in.

When traffic is at its peak, it approaches its ideal state: the on-ramp is fully occupied with cars (no space wasted), and everyone's driving at basically the same constant speed up to the merge point, where everyone neatly and cleanly merges, alternating lanes. But if traffic is not at its peak, zippering doesn't require you to race ahead to merge point.

It wouldn't hurt anyone if you did that, of course. Same for everyone else waiting to merge. But there really are lots of people (where I drive at least) who are already on the road/freeway/whatever, who will move onto the (about to end) merging lane, race up to the end of it and force themselves back on the road, a few cars ahead. I've seen so many bad traffic jams made so much worse by people doing this. It makes a bad situation so much worse for everyone. Those are the people the straddlers are guarding against, not the well-mannered zipperers trying to do the right thing.