Hi Pascal,
I believe that is actually worse. It introduces unwanted faces where points don’t lie on the symmetry axis. See screenshot below. It’s a similar model as above recreated with Rhino SubD tools. I moved a point deliberately away from the symmetry plane and it introduced a new face down the centerline.
For me it would be more intuitive to mirror and fix gaps if any exist. Either by running setpt first or welding disconnected points after running the refelect command.
