Well, any time you'd rather have speed over completeness. Maybe you're aggregating tweets from the Twitter API and if the occasional one goes missing, it's not a big deal, or perhaps you can grab it on the next update. Maybe you're generating a real-time stats dashboard for your site and if one pageview gets lost every million, it's not a big deal.
Look, I agree that in most cases you probably want to do everything you can to make your data 100% complete. But failed writes should be really rare, and there are plenty of times I'd trade the rare missing write for cheaper/faster database servers.
The way MongoDB is designed, this would be outside the scope of a driver. A storage library that's based on the ruby MongoDB driver could certainly do it, though. That's what the OP ideally should have been using. In fact MongoDB would be a good choice for his use case, if he would switch to a 64-bit VM and handle error conditions (heh).