the canvas when new is stiff just like anything, leather or otherwise, that hasn't been broken in. whether or not it softens depends on the bag. the ones that are more structured (like the luco) will maintain their shape (unless of course you stomp on it with your stilettos), while a soft bag (like the speedy) will get droopy over time. the monogram wears remarkably well though. I have a key case that is nearly 10 years old and I swear it looks new.
as for whether or not you think the canvas is worth the $$, it's a personal choice. I personally don't like anything monogramed, but if I had to choose one I would choose LV. much more durable than say gucci. and frankly more durable than some very pricey leather bags, like balenciaga. and I would assume they started using the canvas because it was more durable. but who cares, right? you either like it or you don't.