Something I find really useful for sharpening rollerski ferrules is a Dremel with a diamond-disc. A lot of people will have a Dremel (or other rotary tool), but don't want a full-blown bench grinder. The diamond disc cuts through the ferrule tips like butter. No exaggeration, it sharpens the tips in a few seconds! Another great advantage is that you can take the Dremel with you on holidays.
Here's some pictures showing how to use a Dremel tool:
Despite no sparks, my advice is to wear safety goggles (I wore my cycling/rollerskiiing Oakleys to protect the eyes from flying debris).
I normally sharpen the ferrules when they are mounted on the poles. The tips are off in the photographs because I just got back from holidays and had to take the tips off for the flight...