Because it's known to be ... iffy (seriously, it's hit or miss), the call is to NOT put that in.
There are also occasions when the plugin developer does update the readme with the appropriate compatibility data but that readme isn't parsed correct by the Plugin repo. Or the updated version is delayed by a week or more.
It would give the impression that because a plugin hasn't been updated since 2.9 it's no longer useful, when that isn't always the case.
Seconded. I've been using one plugin for years (literally). It's never been updated. Nor does it need to be. It's simple and has always done its job without a problem from 1.x through to 3.1
Finally, even if you manage to get all of the compatibility data in place, there's still no absolute guarantee that A.N. Plugin will work properly on your server with your theme, your plugin set and your version of WP. It's bad enough trying to get feedback & bug reports on core WP upgrade release candidates over a range of environments. It's ten times worse trying to get any feedback on a plugin beta release. So, at the end of the day, all that the compatibility data can tell you is that the developer has tested his/her plugin against WP x.x on their own development server and, in some really lucky cases, maybe another 1 or two different servers. But nothing that, ultimately, guarantees anything.