What are you shooting? Are you planing on strapping it on and walking around? Stability would be a real problem even with IBIS.
Au contrare. When I use a TLR (or a Hasselblad 500), I put the strap around my neck and the camera sits roughly in the middle of my chest or a bit lower. It is rock steady and very easy to maneuver. Much steadier than a DSLR held up to my eye as it has a much more solid brace (my chest).
Great for crowd shots (and remember, you can be facing in a different direction than the camera is).
Adding IBIS would simply make it even better. It would eliminate the small motion from breathing, or the motion of the platform (boat, train, etc.)
In a crowd, one can hold it over your head inverted and get the shot (flippy/tilty screens are good for this these days, and can also be used to partially simulate the handling of the LTR - except that the body of camera isn't solid against your chest, negating that advantage).
Ergonomically it simply offers a number of benefits. And if you look up the SL3003, you'll see that Rollei covered all the bases, by providing not only a top down view but one of the back as well, so you could use it both in the mode I describe or as an SLR held to your eye and looking through the back.
Pretty impressive for a optical viewfinder.
And the camera hanging right in the middle of your chest, you can add a second strap around your back so it doesn't bounce when you're running. Photojournalists in unpleasant situations should take note of what their predecessors did to survive.
