The idea behind camnets is to break up your sillhouette as well as provide cover from observation... from a far enough distance, and assuming it's well placed, it'll hide the vehicle, but under direct observation, the advantage of camnets is that it hides what's under the net... in theory, you could hide the nature of what's being hidden, such that your enemy doesn't know if it's say, a tank, a radio truck, or a dummy posistion (An empty space with a cam net over it).
I can't see a cam net that hugs the body of the vehicle serving any real purpose, except to break up texture, I can really see it being a hinderance more then an asset on a vehicle that's moving/loading and off-loading troops (Ever get caught in a cam net? Then you know what I mean).
That being said, purpose built cam nets/fixing points for windows would be lovely, but hessian is reasonably quick, and having a stock of hessian on hand is useful, because it's mulit-purpose.