I would not recommend frequently grinding your skis, lightly or otherwise. Once, maybe twice a season or less, but to each his own.
If you are a recreational skier you are only shortening the life span of your equipment. If you are racing you're starting from scratch after any grind with regards to getting your skis fast, and ready to race.
I don't know what you mean by lightly, but even with only one pass 1 of 2 things is happening, either you're changing, most likely convoluting, the existing structure with new structure on top, or you're taking down the structure with each pass. Either way you're changing the existing base, as well as base edge bevel, and this now needs to be corrected. If your grind is done properly in the first place the ski should be flat, smooth, and stay that way for a long time.
As far as edge machines, it all depends on what it is and who is operating it. If its a snowglide or a ceramic disc machine it will remove fractions of a mm while cutting a fresh bevel. If its a sanding belt it depends on the grit, speed, lubrication, and operator.
Edited by SmoothRides - 3/29/13 at 12:59am