Some horses tend to be more tolerant than others; rather than buck our butts off, they try to compensate for our less-than-ideal riding form. Just because a horse can and does compensate, it doesn't that it is good for the horse.
I guess it depends one what one considers a "good" rider. Someone who can ride like the wind may be considered a good rider or someone who can ride a bucking bronco for seven seconds without falling off. Many consider a good rider as someone who can ride at an Olympic level. However, it is not and should not be limited as such. Since we a speaking about "form", I, personally, consider a "good" rider as someone who is balanced and with the horse, yet stays out of the horse's way as much as possible - being an "easy" load. It also depends on what discipline you do. A barrel racer and a dressage rider ride differently, and a casual rider rides differently than someone who shows. Regardless, the goal should remain the same. Of course, everyone makes mistakes and has errors in their riding, but we should try our best and not rely on the horse as simply transportation or a nice place to sit.
"I hear and I forget. I see and I remember. I do and I understand." - Confucius