Three things.... 1) Saves on the site get auto deleted anytime a game gets updated 2) a game could be removed at any time 3) Joiplay app for Android means that Android users can play HTMLs that are stored locally.
Not hard to see why I don't bother trying a game unless I can download it. Not going to play for two days just to have to start over, or worse... discover that the whole game is missing. It's rare, but it has happened.