Nearly useless I'd have to say. To easy to be picked off by a bowman or someone with a musket. Also the battlefield would have to be near a area for the glider to be used such as near a hill. Sorry about that. That's just my thought though.
Balloons were used on (or over) the battlefield as early as 1794, and they weren't shot out of the sky immediately, and they were static targets, although admittedly they were normally deployed behind the front line. Does 1794 count as Early Modern?
Maybe a use would be for message carrying? Just an idea, but I can already see problems with it, so I might agree that they wouldn't have made much of a practical difference.
I feel like you'd need modern material science to get any use out of a glider with no way to get it up into the air. Though I'll admit the idea of Tibetans swooping around the Himalayas by dropping off cliffs and using differential wind speeds around the mountains to soar is pretty cool.
Edit: On further research you could theoretically get paragliders working fairly early, although I imagine the cost of a real silk glider would be prohibitive. It would also require a decent understanding of aeronautics, but I don't think it's necessarily ASB.