Hi jneutron,
In your example it looks like you are making mytr's point for him.
You wrote:"and when the water inside exceeds the dew point of the exhaust, the condensation dissapears"
You are incorrect.
Mtry assumed incorrectly, that the tank surface temperature will remain high enough to prevent condensation. That is an incorrect assumption, and one that the plumber cannot arbitrarily make.
The ONLY time I have had unlimited hot water is with a direct fired oil burner, and that luxury decays with time as scale builds up on the inside of the exchanger over time as well as soot and oxide on the outer surfaces. Typical 40 gallon gas or electric hot water heaters can be run cold, especially if there are no water restrictors in the shower head, if you have a jacuzzi type tub, or if you have several teenage girls.
If you ever have had occasion to run out of hot water (I believe all humans have at one time or another), why would you assume that the tank wall temperature remained at 120 to 140 F????? Think about it, it's water cooled...from a source that's typically at 50 F.
Note: It strikes me that most of you may not know what dewpoint really is. It is a measure of the humidity content of the air, it's just represented as a temperature. If the dewpoint of the air is 60 F, that just means that there is enough moisture in the air that a surface at 60 F will condense moisture, but above that it will not. If you increase the humidity, the dewpoint temperature will increase.
In my example, the result of combustion of gas on the stove creates an exhaust stream which has ADDITIONAL moisture within...one of the byproducts of gas burning is water... The dew point of the exhaust of the gas is higher as a result. This means that even though the pot temperature exceeds the dewpoint of the kitchen air, it is below that of the moisture laden gas stream combustion products. For a simpler example of this, slowly blow hot breath on a mirror... before you do so, there is no visible condensation on the surface, when you breath, there is. This is because the dew point of your breath is above the surface temperature of the mirror.
I say:So in my opinion, there is no rust without moister!
Again, that is incorrect. Rust is iron oxide. It requires only oxygen to grow, air is 1/5th oxygen. A wet one does indeed accelerate the process, and that acceleration is what the plumber wished to prevent. And not only does water accelerate the process, water at 120 to 140 F REALLY accelerates the process until the heat dries out the rest of the water. I would worry about the weld seams the most, as the tank metal immediately next to the weld bead will be the first place rust occurs. This because of the temperature the metal went through during the welding. Even stainless can rust at the welds because of the weld heat driving carbon.
As to your "old wives tale"...Working with fiberglass cloth is indeed messy, aggravating, problematic with regard to breathing concerns, and something I agree most people would prefer not to do.
But not the impetus for the plumber's explanation. It is not a conspiracy.
Cheers, John