Been a subject of great debate recently, and it's now getting attention here:
http://mu.wordpress.org/forums/topic.php?id=8169
Having object cache as a plugin on a WP install makes sense. It's one site, and usually not hit enough to be that big of a problem.
Enter MU, and serving tons of blogs. Every little bit helps, and object cache was definitely helping more than hurting. Yes, there have been a few issues with it, but they can be fixed (widget updating, post/page save and continue editing, etc).
With MU it just doesn't make sense to dump it as a plugin. The file is already there, and it makes more sense to have it in the core so that there is one less file include.