Current browsers do treat <object> differently based on type. For example, if an <object> has a type attribute in the set "application/foobar" or "video/foobar" or "audio/foobar" or "foobar/foobar" ELinks stable will stick a link to it above its rendition of the fallback content. With no type attribute or "image/foobar", it shows the fallback but no link. With "text/foobar" it treats the <object> as an iframe but fails to parse following content correctly. It's hard to see this example, at least, as a demonstration of the utility of content types.
-- Benjamin Hawkes-Lewis