[practicemaths]

If there is an appropriate gap it is fine. However in my experience many people in the zipper lane merge without regard to what the traffic in the lane they're merging into is doing. Thus forcing people to slam on breaks and slows down the overall flow of traffic.

[jstanley]

If you don't leave a gap then you're forcing the people in the other lane to slow down and disrupt the flow of traffic.

It is a mistake to think that one of the two input lanes is more entitled to the output lane than the other. 2 lanes go down to 1, neither is more privileged than the other, cars should merge in turn.

[practicemaths]

If there isn't a gap then don't force a merge. Otherwise you're the one driving unsafely and causing further traffic congestion.

Your comment makes less sense when the merging lane has no one else it as well. That means the car in it that is merging over has plenty of opportunities to merge safely without causing traffic congestion however they don't.

[nkrisc]

If there’s not an appropriate gap then you are also at fault. You should always have an appropriate gap between you and the car in front of you.