A stone grinder is used to both flatten a base and put structure in it. On a manual machine the structure is achieved by the operator running a diamond tipped tool across the wheel to cut small grooves in it. As well as varying these “grooves”, the operator can generally vary the lateral rate (the rate at which the ski moves left and right across the stone as it feeds through it), the feed rate, and the stone speed. As a result it does take quite a skilled operator to do a good grind.
Stone grinding machines, even the most basic, are quite expensive, so many shops may only use a belt. Personally I wouldn’t dream of putting my skis anywhere near a belt.
You can get more information on stone grinders by going to http://www.wintersteiger.com/
there you see some amazing robotic tuners. A friend went to a Wintersteiger demonstration in Europe where they took an old, but otherwise good, ski and purposely incorrectly programmed the machine. Went in ok, came out without any base left at all! Good for the “wow” factor, but generally the more automated the machine, the less reliant it is on a skilled operator (they can also do all sorts of fancy structure patterns). Moral of the story, find yourself somebody who can do a really good grind (and they can be difficult to find) and worship them
One final thing. In addition to removing base material, which is only finite, you also remove the layer of wax contained within it. Assuming no damage, and with the appropriate structure for similar snow conditions, a ski will get faster with use due to the absorption of wax within the base, and the removal of micro P-Tex hairs each time you scrape. Once you do a grind you go back to square one.