Though the subject has been more than a bit controversial as to what exactly was causing the premature failure of mainshaft bearing with heavy 3D - extreme thrust loads on the bearings vs. extreme side loads - a significant amount of testing by Todd and Jason suggested that by spreading the side loads of the mainshaft across multiple bearing surfaces, bearing life was markely increased. The highest failure rate was seen in the bearing in the upper bearing block, so that's where the extra bearing was applied.

Hopes this clarifies things a bit.

Ben Minor