Just ran into a situation where I would have liked this yesterday! (Got an STL from http://3dp.rocks/lithophane/ and realized it was a mirror image when I got it into S3D).
So many times Ive remembered too late that S3d doesn't have this basic function and had to go save a separate mirrored file in sketchup. Makes me grumble every-time. MS paint has this.
Just curious - what are some typical situations where you guys need to use this feature? I'm not sure I've ever had to mirror a model because either I designed it myself or I downloaded it from the thingiverse where most of them are already setup for 3D printing in the correct orientation. Just curious to hear some scenarios where this would be helpful