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Please help me get this straight in my head. The road code seems to dodge the hard question here. Where the car entering the motorway system has to cross a dotted line the implication is that it is changing lanes and should give the same respect to vehicles already using the motorway lane as is due to other users in any lane changing maneuver. Basically, the on-ramp vehicle has no rights to claim space in the motorway lane if it in anyway affects the safety of other road users. This would include entering the safe stopping distance in front of any other car. If the merge-like-a-zip practice is used it is entirely at the discretion of the traffic already in motorway lanes and entering traffic may not assume this. Of course, it is good if the motorway driver can be courteous and adjust his speed to enable on-ramp traffic to enter conveniently but there are times, particularly in dense fast traffic where this is not safe and may also initiate flow irregularity and tailbacks. In talking about "merge like a zip" it should be stressed that this works best in merge situations where there is no lane marking line to be crossed, e.g. where two lanes simply become one lane and there is no "dominant" traffic stream. Am I right or am I wrong?

The "merge like a zip" method of traffic flow can work under both conditions you refer to. However, when lines are present, the traffic on the motorway would have priority.
As you say, courtessy is the key.