Dear Bruce: As an engineering type, I agree that this will work if executed properly. On a small shed with plywood roof sheathing and no collar ties, the plywood is acting as your A-B and B-C members. In a larger building, your member C-D and the corresponding member at the opposite side would have to be designed as collar ties resisting a significant amount of tension. Also member A-C would experience a significant amount of tension. This scheme is not typically done for two reasons:
1. The connections must be engineered to accomodate the large loads and given geometry.
2. There is a lack of redundancy. If there is a failure of either tie, the roof will soon follow.
Given these drawbacks, it can be done if the situation warrants it. It may be cheaper though to install a monster ridge beam.
