PHil,
It always boils down to this. People will buy the lens that they want, and if it's a choice made after objectively testing and assessing the lens performance, then it's a good choice. On the other hand, a lens bought purely on advice (often specious), particularly from forum advice which is not always objective or is plain wrong, is a purchase made inadvisably. I strongly believe that this is the reason that a lot of the complaints made.
A few pro's as you say may be ignorant, but their picture editors are not, and apart from a few 'scoop' pictures, where poor technique may be tolerated, picture IQ, sharpness etc, have to be spot on. I'd like to bet that pros who have to buy and pay for their equipment are not ignorant either about quality or the IQ they expect from their purchase
I'd also like to bet that less than 001% of people who buy a lens ever read a forum, and are 100% happy with their purchase.