I believe a Change Log should be part of the theme package because it's like software, every change should be noted, especially for those who do check them out. There's also instances where a web designer or webmaster is managing a client's website; should there be an update, it's important to see what changes were made.

I've also seen themes get radical changes, so it wold be nice to know what was changed. As one who designs and develops WordPress themes, I like to include a changelog, in both my free (at wordpress.org) and commercial themes.

For the most part, the average user of themes from the repository probably does not read or even realize there's a change log, but again, there are some who know it and read them.