You do have a point. In theory. I haven't run across a case in practice yet where you could expect both the scaled down and original size image to be viewed with that kind of certainty.
As for thumbnails, I am on a slowish connection and I consider it rude if someone is too lazy to put up scaled down thumbs to serve; more than likely I'm not going to bother waiting for the gallery to load.
The only case where it is realistically sensible to use IMG attributes is when the same image is used in various scaling factors on the same page - which is usually the scaled transparent pixel scaler..
Makeshifts last the longest.