If we were talking furniture, mortise and tenon would have a clear advantage, but in this application dowels are not that far behind. The time savings is probably more than worth the reduced strength; of all the SOF's build with doweled deck beams I've never heard of any failures. Dowels are certainly more approachable than mortise and tenon, especially for the novice woodworker.

: Any strength or time saving advantage for deck beams?