I find the save dialog useless for web browsers as well, but I think preventing its use is a bad idea in general. Overriding the browser's shortcut is uncomfortable. For example, wordpress likes to capture cmd+<number> to change the font style, but that's how I usually change the active tab. It also disables ctrl+tab, the other way I use to escape while the text area is active.
People use their browsers and have their workflow in them. Breaking them needs to have a really good excuse. http://xkcd.com/1172/