That's not my experience at all. When I ask them to track down the root cause of a bug about 80% of the time they reply with a few sentences correctly identifying the source of the bug.

1/5 times the get it wrong and I might waste a minute or two confirming they they missed. I can live with those odds.

I'm assuming you delegate for most of your bugs? I only ask when I'm stumped and at that point it's very prone to generating false positives.