I've NEVER heard of a Colt SAA being carried in a half cock position.
Neither have I. But the op didn't mention SAAs. He was referring to cap-and-ball revolvers. Even though he didn't specifically say so in his first post, I assumed as much.
Others went on to mention the SAA, and just went with the discussion of sear design in general, the quarter cock and half cock sears being essentially the same in construction and strength. I suspect we all know that on a SAA it is the quarter cock notch that is/was used as a "safety".