Might make sense for professional, who's going to sharpen chain off the
bar. Lots of chains.

For home use, this file-guide has worked great for me for ~25 yrs-
http://www.right-tool.com/right-tool/filenjoint.html. Fringe-benefits:
works out in the woods, is not inclined to removing significant
fraction of cutter, and works on mounted chain (for lack of better
phrase.)

J