One of the main reasons behind digging over the idlers is that in heavy digging the load is transfered from the pads to the links/rails then directly to the idlers which have support on both ends of them, where as over the sprockets the load is on the bushing and then to the sprocket, the rails take no load at all, if the bushes are getting thin they will end up cracking/breaking etc as they are not as strong as the rails are, plus the sprocket has no outer support as the idler does. There are probably other reasons as well but that is one that comes to mind. Cheers RDG.