The main advantage of the bench grinders is speed. However, in my experience even a high-end grinder in the hands of an experienced operator does not do as good a job as a file in the hands of someone experienced in it's use.
Having said that, I've met dozens of people who claim to be very good at free-hand filing (using just a bare round file with no guides of any sort). Most of them could make their chains cut better than a dull chain. However, I have met only three who actually were really good at it - they could reliably make their chains cut better than a brand new out of the box chain. (Sadly, I am not one of those three. I've never put the effort into that.)
I have never felt the need to learn with just a bare round file, since I can get better than new results with a round file using a roller guide or the Oregon-type file guide that clips onto the file and occasionally touching up the rakers with a progressive depth gauge guide. I've also tried the Stihl 2-in-1 file guide. I found it fast and easy to learn. Someone new to sharpening can learn to get good results quickly, and it saves some time by not needing to separately address the rakers. I prefer the Roller or the Oregon guides, since they allow the use of a progressive depth gauge tool on the rakers (the Stihl 2-in-1 uses a non-progressive method for setting the rakers). This gives a slight edge in cutting speed, and means that you don't have to worry about keeping your cutters all the same length.
When using a guide, you do have to put up with the sneers of those who file with a bare round file and their comments about needing "training wheels". I've shut a few of them up over the years by challenging them to sharpen their saw and I'll sharpen mine and we'll see how they compare when cutting for speed and straightness of cut. The challenge generally worked out well for me - except when I finally ran into one of the three I mentioned earlier who was in fact really good with a bare file. (One of the loudest self-proclaimed bare file experts was embarrassed when he found out that my 57cc saw had just beaten out his 70cc saw - but then he was simply not very good at sharpening at all, he just thought he was). It's not that I claim any particular genius when sharpening with a guide, it just that it is significantly easier to get good results with a guide than without. Someone new to sharpening can get acceptable results using a guide with just a short training session and a little bit of practice. With just a bit more practice and maybe some pointers from someone with some experience, they can match new chain performance.
Mine's made by Oregon, I buy a box of 9 for about 15 bucks. Yet all of the other methods mentioned above work for the user, otherwise they wouldn't be posting about it. For best results put it in a vise and use a file guide when it starts to get out of whack. I usually run the file across the teeth about every other tank of gas though, and take the rakers down about every 5th sharpening.
