Last week, I was in the middle of a failed release that needed to be rolled-back. After rolling-back the code on the live production server, we found that there were issues. I was not told of a release a few weeks earlier where the developer placed a single module on the server without installing an entire release.
So on the next release last week, when the deploy blew away the developer's changes, the only way to get back the developer's changes was on the peer server. With a single server environment, this would not have been possible.
Yet again, this shows that partial releases have no place in live server environments.