One of the issues with sharpening is that not all files are 'the same', and not all grinding wheels are 'the same'. There are trade-offs between speed of cut, smoothness of finish, life of the file/grinding wheel, etc. The different grinders you mention might make less of a difference (assuming that they run true) than the wheels used. Then, of course, there is the skill of the user!
I have watched this thread with interest, and would like the chance to see/try one of the Timberline sharpeners some day. It appears to provide a finished tooth that posters here have been impressed with, without too much fuss. But you are also limited to a few, specific choices.
With a grinder I can change a number of the sharpening angles (if I want/need to) for different chains or different types of cutting. I can put on different wheels (although, some are hard to get) to grind quicker, or to provide a smoother finish, if that is important. Same with filing - I could start with a coarse file for really rocked chain, then finish with a smoother file. But this all assumes that I want to do this or that it is important.
My bottom line has always been for people to find something that works for them. I have only see positive things posted in this thread, so I have to assume that it works for a lot of guys.
Philbert