Reading through accident reports, these things seem to involved in a fair bit of death and destruction. There seem to be two major classes of failures: the block was improperly rigged (inexperience and complexity making it difficult to inspect) the "block" has no mechanical locking mechanism and is going to fail in certain circumstances (ex: jester) So given the above and the assumption that most people don't want to kill themselves or others, a couple questions: Why are these things even used outside of class C canyons? The only two cases that I can think of are for beginner groups or when you can't see the bottom. For beginners, I would hope that they have been prepared enough before being put in a canyon that lowering events are rare, and convert to lower (or even cut and lower) would be sufficient. When you cannot see the bottom, it would seem that a double rope rappel or a biner block, toss, and adjust would be nearly as efficient but significantly safer due to lower complexity of the system. Why are device-specific releasable blocks popular, when the munter-mule-overhand is sufficient? I would hope that anyone leading a group would already be familiar with the MMO as a backup in case of a lost rappel device. More importantly, others with experience should be able to inspect an MMO. Even the basic figure 8 block has tons of variations (many of which are non-locking, and thus insecure in certain situations) that make inspection tricky. I wouldn't trust my life to something that I couldn't inspect without a backup.