I am sorry about your roo and black pullet. If he was ill wouldn't it show on the test? Did they give you "cause of death"? I think you are right - its because we count every SFH that they are more noticeable.
I have had only one bird tested - that was a Show Girl given to me because she was acting "off" - and when she died I wanted to make sure she didn't carry anything that would affect the rest of my flock. She was completely in quarantine - no contact with any other bird, fed and handled last and changed clothes. She went downhill very quickly, but no sign of outward illness except she refused to eat or drink and would fight any force feeding.
The necropsy report came back chronic inflamation of the bronchial tubes and feeding "tube" (forgot exact name) and bacterial and fungal growths inside her lungs and airways. It was very specific on cause of death (dehydration and starvation because she would not eat or drink because it hurt). I don't know if it was because I requested cause of death or what, but it wasn't just "not MG, Salmonella, etc.." it told me what happened. That information was included, but that was not the end of the test. I was able to contact the previous owner and she has since changed the living situation for the rest of her birds, hopefully no more will get sick like that.
I have only had one other drop-dead bird - it was a juvie Mottled Houdan last year and he didn't look well in the morning, gone by noon. If I had known about the testing at that time I would have sent him in. I only found out about it the beginning of this year. I suspected heart trouble, but had no way to test.
I agree - it is very useful to know what is wrong. I am not sure why they don't give you any more specifics - perhaps you have to request it? Mine was done by UC Davis, perhaps they use the birds as teaching tools so they do a complete exam, more than you would get from a cursory exam.