This is an interesting topic, so I will reply.
First of all, I am currently of the opinion that Meyer's dusack is used to teach one handed cutting techniques for all types of weapons of the period. He often references the dusack section when discussing cutting with the rapier.
In our experience here, we train beginners on cutting with the dusack. In fact its almost exclusively used as a cutting weapon, until second rank. Through this I have noticed that my students quickly adapt to cutting with the rapier, and most importantly understand the basic universal concepts of throwing a cut, and how to effectively generate power.
In my personal experience, by training with the dusack I have found my cutting is more crisp right across the board, regardless of the weapon I am using.
Also we must ask the obvious question. "If messers were common place in this time period, why would meyer not include them in some fashion or form in his book when he includes all of the other common weapons?"
This question can also be asked of Mair, as he also has an extensive dusack section.
Especially when we look at the art of the time, Messers were everywhere. They were even depicted as a weapon of choice in religious scenes... ie when simon cuts off the ear of Malchus in defense of jesus. http://www.lib-art.com/artgallery/3618- ... huber.html
- This is just one of the many images I have found that show the messer in period art.
I believe the answer is obvious. The dusack, as the equivilant bouting weapon for the messer, eventually became the way to teach people how to fight with a messer.
My present interpretation of the meyer dusack/messer connection is: If one can apply cuts and thrusts while transitioning through meyers dusack guards while understanding how to skillfully apply concepts of measure and time, and understand how to effectively and efficiently generate power, then one is able to effectively fight with a messer.