yea i would never use something like that with a glidecam, i only said that because the guy who quoted me argued my point by saying you wouldn't need IS, but I don't think he realizes how noticeable camera shake would be at 70mm or higher. with my glidecam, i only ever use my 17-55mm f/2.8 at 17mm.
As for advantage of the 2.8, I always shoot wide open since I love getting the most shallow depth of field possible, even if i'm doing follow cams. So 2.8 would appeal to me as you can make it a little more shallow, but to OP, if you're going to be shooting above f/4, especially for video, then you might not need the 2.8
The joy I get from skiing...that's worth dying for. - CR Johnson