Firstly I don't believe the lure is ever stationary.  I believe at the instant of the strike (when the fish takes hold of the lure) the lures drag / resistant force effectively becomes zero.  We are talking fractions of a second here so in practical terms perhaps this discussion is nothing more than semantics.

If I understand correctly the purpose of PB's post it was to question whether the drag setting of the reel should be increased when towing lures that offer more resistance than others.  In short my opinion would be no.

In the next few seconds, as the lure slows to a stop, its drag reduces to 0 & the marlin is now pulling the full 5kg.

The lures drag / resistant force ceases the moment the fish takes hold of the lure, but more semantics.

When the marlin takes off, and drags the lure (backwards) through the water, the lure drag is added to the reel drag.

I think you are confusing force with reel drag.  The fish can't add drag to the reel drag.  If the fish and trailing lure exerts more force than the drag setting line will come off the reel.

Similar to the principal behind backing off the drag when you have several hundred metres of line out, when the friction of the line going through the water can add several kgs of extra drag, felt at the fish end.

Agree in principle.  The belly in the line can increase the force or resistance over a portion of the line which can result in the line breaking.

If the marlin is swimming faster than the boat was trolling, the drag will be greater too. In the case of the lure, the hooks & trace will have to withstand the extra drag, but the mainline (and anything above the lure) won't.

I'm not sure exactly what you are saying here?

In the case of a lot of line out, drag increases progressively through the mainline as well.

I think you are referring to the fact that the reels drag pressure increases as the spool diameter decreases, if so I agree.

We are mostly on the same page (I think) despite the use of different terminology.

If I understand correctly the purpose of PB's post it was to question whether the drag setting of the reel should be increased when towing lures that offer more resistance than others.  In short my opinion would be no.

and I agree.

The lures drag / resistant force ceases the moment the fish takes hold of the lure, 

Well - no it doesn't. It ceases when the fish stops the lures progress through the water, and restarts when the fish starts towing the lure. OK - lets call this one semantics, although driving the hook home with a heavy drag at the instant of the strike is a well established and (for many) effective way of fishing. The point is that at the instant of the strike, the lure drag is subtracted from the reel drag

I think you are confusing force with reel drag.  The fish can't add drag to the reel drag.  If the fish and trailing lure exerts more force than the drag setting line will come off the reel.

Clumsy wording on my part. No the fish can't add drag to the reel, but the drag the fish feels is the sum of all the drags - reel, lure (+ or -), line friction etc.

Agree in principle.  The belly in the line can increase the force or resistance over a portion of the line which can result in the line breaking.

Yes, but its not just a belly in the line that will do that. Just the friction of several hundred metres of line being dragged in a straight line can add several kgs of drag, even with nothing tied to the end. Anyone who has been spooled and snapped their mainline at the double knows this well. Been there, done that 

I'm not sure exactly what you are saying here?

Drag/velocity relationships are a complicated subject, but as velocity increases, so does drag. Sometimes the relationship is linear (double the speed, double the drag) but sometimes the drag increases by the square of the velocity. I'm getting out of my depth here, but even if it is just a linear progression, that 3kg drag from the lure quickly becomes 6kg if the fish takes off at double the boat's trolling speed. The extra is not transferred to the mainline, but is felt by the leader and hook, so becomes relevant if using lighter gauge hooks.

I think you are referring to the fact that the reels drag pressure increases as the spool diameter decreases, if so I agree.

I was actually referring to the friction of the line in the water, but yes the reducing lever length as the spool empties also requires more force to overcome the same drag setting. I have no idea how to quantify that, but I'm sure engineers have calculated it.

Hey MF - why don't you ask us an easy one next time, like 'should I use single or double hooks'