Tim --
The use of a filler in the canvas covering certainly adds weight, but it also adds considerable durability.
I would be interested to know just how filled canvas came to be developed and used for canoe covering, and who did it first. Canvas-covered wood canoes were developed at just about the same time as linoleum was introduced into this country, in the 1870's, and the make-up of linoleum and filled canvas are quite similar. The people who developed canvas covered wood canoes were tinkerers and innovators, and it is not surprising to me that they would have looked to other technologies of their time to improve upon their idea of replacing birch bark with canvas.
Real linoleum (not the modern sheet vinyl commonly referred to as linoleum) is canvas filled and covered with linseed oil mixed with various filler ingredients -- pine rosin, ground cork dust, wood flour, mineral fillers such as calcium carbonate -- were all tried at various times, in various combinations.
http://en.wikipedia.org/wiki/Linoleum . It seems that there was quite a bit of experimenting in England and this country with how to make a durable material from canvas and linseed oil.
Also, floor cloths made from canvas painted with (linseed) oil-based paint had been used in this country since at least colonial times to protect floors, and light-weight canvas or linen fabric coated with linseed oil -- oil cloth -- was commonly used for waterproof table cloths into the 1950's. I remember as grammar school student in the 1950's that I and my classmates were required to have a piece of oil cloth to cover our desks at lunchtime. Today vinyl has replaced oil cloth, just as it has largely replaced linoleum. Linoleum itself, after its introduction, quickly replaced floor cloths of painted canvas, which had been in common use in houses to protect floors since at least colonial times -- linoleum was much more durable.
As you have experienced, simply painting a canvas covering produces a water-tight hull skin. And the canoe would probably paddle just about the same without the filler. But filled canvas is much more resistant to abrasion and tearing, just as linoleum is much more durable than a simple oil cloth or a painted floor cloth.
Greg