My concern was with the reliability of the product.
I wouldn't be so quick to assume there is a rule for fit that applies to all people. There are enough threads running right now about how unhappy some people are with the popular bike-fitting methods. I agree with the folks who basically say "Use what works."
Some people seem to be assuming I want to alter the q-factor due to a knee problem. I have no knee problems. I just know that I cycle with less discomfort on my wide Q-factor MTB, even though the angles, reach, saddle height, etc. are about the same on each bike.
The only difference I can find is that the pedals are about 1/2" farther out on each side on the mountain bike than on the road bike, and I really like it that way. It feels like I'm fully using my legs and hips to drive the pedals, not fighting with the pedals like I do on other bikes.
I'm not going to buy the pedal extenders anytime soon. They cost way too much and push the pedals out a little too far.
An archived article on VeloNews by Zinn indicates you can make adjustments with a few washers. I'll make gradual modifications and see how it goes. I know I can't push the pedals out a full 1/2", but there is probably some point around 1/4" that will provide a comfortable fit without compromising the pedal/crank interface.
And I'm just a recreational cyclist. I'll rarely ride more than 100 miles a week.