When I bought my SLR650 last year, I immediately fitted it with a screen as I'd become soft after too may years with fairings. I've always felt it was a bit "nervous" especially when coming up behind lorries etc.
So today as it's nice & warm I thought I'd try it without and the feel is so much better (although draughtier, but with the compensation that there's less engine noise...) It was rock steady even coming up behind artics at 70 mph or so which would have scared me a bit with the screen fitted.
I think the problem is that as it fits to the bars, any turbulent wind flow tends to catch it and exaggerate and movement as it acts like a sail.