I've never had anyone on the back on my Versys since getting my 37L Trax cases installed, but there seems to be enough room for a comfortable ride. I hopped on the back of mine just now to check it out and found that the cases don't get in the way of legs or anything at all. If anything, they might provide a little extra comfort because they give you something else to rest your hands on. The tops are a little lower than the rear grab handles though, so you might need someone with long arms to take advantage of this.
2012 Kawasaki Versys 650 - 20,500 miles
2000 Volvo V70XC 2.4T - 258,000 miles