Mechanics usually have a very poor understanding of engineering principals because their training doesn't include it. They draw their conclusions from personal observations working on used cars, and with lack of fundamental engineering training it is no surprise that these conclusions are usually wrong, although based on valid observations. That's IMO, not an indication that there people are bad mechanics.
Is it really their fault for not recommending something that, from their experience, has a good chance of a come back and blaming them for a bad tranny?
It's easy to play the know it all on BITOG with google at your fingertips.