Here's our accidental gybe story (the most recent one, that is): The first year we got our new 44' blue water boat we sailed around Vancouver Island. Rounding the Brooks Penninsula we were flying downwind in 30-35 knots - I saw 14knots briefly on the knotmeter. Boom! - accidental gybe, broke one of the mainsheet blocks and damn near ripped the track off the cabin top.
We had a Dutchman, but I would have to say it wasn't set up tight enough to do its job. Only a few other contributing factors, though: sailing overcanvassed (one reef in the main, should have been 2 or 3), nearly out of control, etc.
Since then we have sailed across the South Pacific, logging 13,000 mostly downwind miles. We still use the Dutchman, but now have and use preventers run from stout padeyes at the end of the boom, forward to the peak and then back to
Lewmar linestoppers. The Dutchman is used to control intended gybes. It may be belt and suspenders, but we think they are both needed when making long passages downwind under wind vane steering.