With all due respect, OMS is going to tell you what they want you to hear. They will tell you it is all human error, and even with a double barrel o-ring, you can have human error. However, wouldn't you like to reduce the risk of error? Wouldn't you like to know that if you smack a ceiling of a cave, your manifold has a better chance of not letting go? I know I would.
As for needing to get the halcyon, don't bother. It is too expensive. The SeaElite 300bar manifold is the same thing, even has rubber knobs.
If you want to know why DIR is a better system for most types of cave diving, then the answers are out there. However, such a broad question like the one you ask is very hard to answer. Read up on DIR, and then when you understand the system better, you will be able to answer more specific questions that are easier and shorter to answer.
As for my feelings on DIR, I am not DIR. I dive a DIR style gear configuration, but I have not taken any DIRF training, and I do not conform completely. I am simply a person who sees the benefits of the system.